Junk Charts

Message-first visualization

Sneaky Pete via Twitter sent me the following chart, asking for guidance:

This is a pretty standard dataset, frequently used in industry. It shows a breakdown of a company's profit by business unit, here classified by "state". The profit projection for the next year is measured on both absolute dollar terms and year-on-year growth.

Since those two metrics have completely different scales, in both magnitude and unit, it is common to use dual axes. In the case of the Economist, they don't use dual axes; they usually just print the second data series in its own column.

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I first recommended looking at the scatter plot to see if there are any bivariate patterns. In this case, not much insights are provided via the scatter.

From there, I looked at the data again, and ended up with the following pair of bumps charts (slopegraphs):

A key principle I used is message-first. That is to say, the designer should figure out what message s/he wants to convey via the visualization, and then design the visualization to convey that message.

A second key observation is that the business units are divided into two groups, the two large states (A and F) and the small states (B to E). This is a Pareto principle that very often applies to real-world businesses, i.e. a small number of entities contribute most of the revenues (or profits). It is very likely that these businesses are structured to serve the large and small states differently, and so the separation onto two charts mirrors the internal structure.

Then, within each chart, there is a message. For the large states, it looks like state F is projected to overtake state A next year. That is a big deal because we're talking about the largest unit in the entire company.

For the small states, the standout is state B, decidedly more rosy than the other three small states with similar projected growth rates.

Note also I chose to highlight the actual dollar profits, letting the growth rates be implied in the slopes. Usually, executives are much more concerned about hitting a dollar value than a growth rate target. But that, of course, depends on your management's preference.

 

Crazy rich Asians inspire some rich graphics

On the occasion of the hit movie Crazy Rich Asians, the New York Times did a very nice report on Asian immigration in the U.S.

The first two graphics will be of great interest to those who have attended my free dataviz seminar (coming to Lyon, France in October, by the way. Register here.), as it deals with a related issue.

The first chart shows an income gap widening between 1970 and 2016.

This uses a two-lines design in a small-multiples setting. The distance between the two lines is labeled the "income gap". The clear story here is that the income gap is widening over time across the board, but especially rapidly among Asians, and then followed by whites.

The second graphic is a bumps chart (slopegraph) that compares the endpoints of 1970 and 2016, but using an "income ratio" metric, that is to say, the ratio of the 90th-percentile income to the 10th-percentile income.

Asians are still a key story on this chart, as income inequality has ballooned from 6.1 to 10.7. That is where the similarity ends.

Notice how whites now appears at the bottom of the list while blacks shows up as the second "worse" in terms of income inequality. Even though the underlying data are the same, what can be seen in the Bumps chart is hidden in the two-lines design!

In short, the reason is that the scale of the two-lines design is such that the small numbers are squashed. The bottom 10 percent did see an increase in income over time but because those increases pale in comparison to the large incomes, they do not show up.

What else do not show up in the two-lines design? Notice that in 1970, the income ratio for blacks was 9.1, way above other racial groups.

Kudos to the NYT team to realize that the two-lines design provides an incomplete, potentially misleading picture.

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The third chart in the series is a marvellous scatter plot (with one small snafu, which I'd get t0).

What are all the things one can learn from this chart?

• There is, as expected, a strong correlation between having college degrees and earning higher salaries.
• The Asian immigrant population is diverse, from the perspectives of both education attainment and median household income.
• The largest source countries are China, India and the Philippines, followed by Korea and Vietnam.
• The Indian immigrants are on average professionals with college degrees and high salaries, and form an outlier group among the subgroups.

Through careful design decisions, those points are clearly conveyed.

Here's the snafu. The designer forgot to say which year is being depicted. I suspect it is 2016.

Dating the data is very important here because of the following excerpt from the article:

Asian immigrants make up a less monolithic group than they once did. In 1970, Asian immigrants came mostly from East Asia, but South Asian immigrants are fueling the growth that makes Asian-Americans the fastest-expanding group in the country.

This means that a key driver of the rapid increase in income inequality among Asian-Americans is the shift in composition of the ethnicities. More and more South Asian (most of whom are Indians) arrivals push up the education attainment and household income of the average Asian-American. Not only are Indians becoming more numerous, but they are also richer.

An alternative design is to show two bubbles per ethnicity (one for 1970, one for 2016). To reduce clutter, the smaller ethnicites can be aggregated into Other or South Asian Other. This chart may help explain the driver behind the jump in income inequality.

 

 

 

 

 

Two thousand five hundred ways to say the same thing

Wallethub published a credit card debt study, which includes the following map:

Let's describe what's going on here.

The map plots cities (N = 2,562) in the U.S. Each city is represented by a bubble. The color of the bubble ranges from purple to green, encoding the percentile ranking based on the amount of credit card debt that was paid down by consumers. Purple represents 1st percentile, the lowest amount of paydown while green represents 99th percentile, the highest amount of paydown.

The bubble size is encoding exactly the same data, apparently in a coarser gradation. The more purple the color, the smaller the bubble. The more green the color, the larger the bubble.

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The design decisions are baffling.

Purple is more noticeable than the green, but signifies the less important cities, with the lesser paydowns.

With over 2,500 bubbles crowding onto the map, over-plotting is inevitable. The purple bubbles are printed last, dominating the attention but those are the least important cities (1st percentile). The green bubbles, despite being larger, lie underneath the smaller, purple bubbles.

What might be the message of this chart? Our best guess is: the map explores the regional variation in the paydown rate of credit card debt.

The analyst provides all the data beneath the map. 

From this table, we learn that the ranking is not based on total amount of debt paydown, but the amount of paydown per household in each city (last column). That makes sense.

Shouldn't it be ranked by the paydown rate instead of the per-household number? Divide the "Total Credit Card Paydown by City" by "Total Credit Card Debt Q1 2018" should yield the paydown rate. Surprise! This formula yields a column entirely consisting of 4.16%.

What does this mean? They applied the national paydown rate of 4.16% to every one of 2,562 cities in the country. If they had plotted the paydown rate, every city would attain the same color. To create "variability," they plotted the per-household debt paydown amount. Said differently, the color scale encodes not credit card paydown as asserted but amount of credit card debt per household by city.

Here is a scatter plot of the credit card amount against the paydown amount.

A perfect alignment!

This credit card debt paydown map is an example of a QDV chart, in which there isn't a clear question, there is almost no data, and the visual contains several flaws. (See our Trifecta checkup guide.) We are presented 2,562 ways of saying the same thing: 4.16%.

 

P.S. [6/22/2018] Added scatter plot, and cleaned up some language.

 

 

 

Foodies say, add dataviz spice please

This Buzzfeed article proves that foodies love their food served with dataviz (tip: Chris P.). Menus are an undertapped resource when it comes to data visualization.

There are several examples worth discussing.

Venn diagrams are not easy to read, people.

Plus they are hard to construct well... note the asymmetric areas.

Here is one without circles:

Then, I pared it down to its essence:

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This beer map is pretty great:

Some of its virtues:

• The spacious layout utilizing two dimensions, instead of a one-dimensional list of dense text
• Ordering using two dimensions relevant to the decision problem (assuming those two dimensions are the most important for their clients)
• Unconventional, attention-grabbing
• More equitable: different readers will read the chart in different orders. I'll hypothesize that they will end up with a more even distribution of drink orders than with a list in which everyone reads top to bottom

Potential problems:

• Not enough space to explain the drinks. Don't the clients want to know what's in them?
• I wonder how they measured the degree of "classic"-ness.

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This next menu contains an error:

When the drink comes in one size, only one price is listed. If it comes in two sizes, two prices should be listed.

Is the cafe owner shading Americans as not good at math?

Lines, gridlines, reference lines, regression lines, the works

This post is part 2 of an appreciation of the chart project by Google Newslab, advised by Alberto Cairo, on the gender and racial diversity of the newsroom. Part 1 can be read here.

In the previous discussion, I left out the following scatter bubble plot.

This plot is available in two versions, one for gender and one for race. The key question being asked is whether the leadership in the newsroom is more or less diverse than the rest of the staff.

The story appears to be a happy one: in many newsrooms, the leadership roughly reflects the staff in terms of gender distribution (even though both parts of the whole compare disfavorably to the gender ratio in the neighborhoods, as we saw in the previous post.)

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Unfortunately, there are a few execution problems with this scatter plot.

First, take a look at the vertical axis labels on the right side. The labels inform the leadership axis. The mid-point showing 50-50 (parity) is emphasized with the gray band. Around the mid-point, the labels seem out of place. Typically, when the chart contains gridlines, we expect the labels to sit right around each gridline, either on top or just below the line. Here the labels occupy the middle of the space between successive gridlines. On closer inspection, the labels are correctly affixed, and the gridlines  drawn where they are supposed to be. The designer chose to show irregularly spaced labels: from the midpoint, it's a 15% jump on either side, then a 10% jump.

I find this decision confounding. It also seems as if two people have worked on these labels, as there exists two patterns: the first is "X% Leaders are Women", and second is "Y% Female." (Actually, the top and bottom labels are also inconsistent, one using "women" and the other "female".)

The horizontal axis? They left out the labels. Without labels, it is not possible to interpret the chart. Inspecting several conveniently placed data points, I figured that the labels on the six vertical gridlines should be 25%, 35%, ..., 65%, 75%, in essence the same scale as the vertical axis.

Here is the same chart with improved axis labels:

Re-labeling serves up a new issue. The key reference line on this chart isn't the horizontal parity line: it is the 45-degree line, showing that the leadership has the same proprotion of females as the rest of the staff. In the following plot (right side), I added in the 45-degree line. Note that it is positioned awkwardly on top of the grid system. The culprit is the incompatible gridlines.

 

The solution, as shown below, is to shift the vertical gridlines by 5% so that the 45-degree line bisects every grid cell it touches.

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Now that we dealt with the purely visual issues, let me get to a statistical issue that's been troubling me. It's about that yellow line. It's supposed to be a regression line that runs through the points.

Does it appear biased downwards to you? It just seems that there are too many dots above and not enough below. The distance of the furthest points above also appears to be larger than that of the distant points below.

How do we know the line is not correct? Notice that the green 45-degree line goes through the point labeled "AVERAGE." That is the "average" newsroom with the average proportion of female staff and the average proportion of leadership staff. Interestingly, the average falls right on the 45-degree line.

In general, the average does not need to hit the 45-degree line. The average, however, does need to hit the regression line! (For a mathematical explanation, see here.)

Note the corresponding chart for racial diversity has it right. The yellow line does pass through the average point here:

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In practice, how do problems seep into dataviz projects? It's the fact that you don't get to the last chart via a clean, streamlined process but that you pass through a cycle of explore-retrench-synthesize, frequently bouncing ideas between several people, and it's challenging to keep consistency!

And let me repeat my original comment about this project - the key learning here is how they took a complex dataset with many variables, broke it down into multiple parts addressing specific problems, and applied the layering principle to make each part of the project digestible.

 

 

A look at how the New York Times readers look at the others

The above chart, when it was unveiled at the end of November last year, got some mileage on my Twitter feed so it got some attention. A reader, Eric N., didn't like it at all, and I think he has a point.

Here are several debatable design decisions.

The chart uses an inverted axis. A tax cut (negative growth) is shown on the right while a tax increase is shown on the left. This type of inversion has gotten others in trouble before, namely, the controversy over the gun deaths chart (link). The green/red color coding is used to signal the polarity although some will argue this is bad for color-blind readers. The annotation below the axis is probably the reason why I wasn't confused in the first place but the other charts further down the page do not repeat the annotation, and that's where the interpretation of -$2,000 as a tax increase is unnatural!

The chart does not aggregate the data. It plots 25,000 households with 25,000 points. Because of the variance of the data, it's hard to judge trends. It's easy enough to see that there are more green dots than red but how many more? 10 percent, 20 percent, 40 percent? It's also hard to answer any specific questions, say, about households with a certain range of incomes. There are various ways to aggregate the data, such as heatmaps, histograms, and so on.

For those used to looking at scientific charts, the x- and y-axes are reversed. By convention, we'd have put the income ranges on the horizontal axis and the tax changes (the "outcome" variable) on the vertical axis.

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The text labels do not describe the data patterns on the chart so much as they offer additional information. To see this, remove the labels as I have done below. Try adding the labels based on what is shown on the chart.

Perhaps it's possible to illustrate those insights with a set of charts.

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While reading this chart, I kept wondering how those 25,000 households were chosen. This is a sample of  households. The methodology is explained in a footnote, which describes the definition of "middle class" but unfortunately, they forgot to tell us how the 25,000 households were chosen from all such middle-class households.

The decision to omit the households with income below $40,000 needs more explanation as it usurps the household-size adjustment. Also, it's not clear that the impact of the tax bill on the households with incomes between $20-40K can be assumed the same as for those above $40K.

Are the 25,000 households is a simple random sample of all "middle class" households or are they chosen in some ways to represent the relative counts? It's also useful to know if they applied the $40K cutoff before or after selecting the 25,000 households. 

Ironically, the media kit of the Times discloses an affluent readership with median household income of almost $190K so it appears that the majority of readers are not represented in the graphic at all!

 

Excellent visualization of gun violence in American cities

I like the Guardian's feature (undated) on gun violence in American cities a lot.

The following graphic illustrates the situation in Baltimore.

The designer starts by placing where the gun homicides occured in 2015. Then, it leads readers through an exploration of the key factors that might be associated with the spatial distribution of those homicides.

The blue color measures poverty levels. There is a moderate correlation between high numbers of dots (homicides) and deeper blue (poorer). The magenta color measures education attainment and the orange color measures proportion of blacks. In Baltimore, it appears that race is substantially better at explaining the prevalence of homicides.

This work is exemplary because it transcends description (first map) and explores explanations for the spatial pattern. Because three factors are explored together in a small-multiples layout, readers learn that no single factor can explain everything. In addition, we learn that different factors have different degrees of explanatory power.

Attentive readers will also find that the three factors of poverty, education attainment and proportion black are mutually correlated.  Areas with large black populations also tend to be poorer and less educated.

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I also like the introductory section in which a little dose of interactivity is used to sequentially present the four maps, now superimposed. It then becomes possible to comprehend the rest quickly.

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The top section is less successful as proportions are not easily conveyed via dot density maps.

Dropping the map form helps. Here is a draft of what I have in mind. I just pulled some data from online sources at the metropolitan area (MSA) level, and it doesn't have as striking a comparison as the city-level data, it seems.

 

 PS. On Twitter, Aliza tells me the article was dated January 9, 2017.

Getting into the head of the chart designer

When I look at this chart (from Business Insider), I try to understand the decisions made by its designer - which things are important to her/him, and which things are less important.

The chart shows average salaries in the top 2 percent of income earners. The data are split by gender and by state.

First, I notice that the designer chooses to use the map form. This decision suggests that the spatial pattern of top incomes is of top interest to the designer because she/he is willing to accept the map's constraints - namely, the designer loses control of the x and y dimensions, as well as the area and shape of the data containers. For the U.S. state map, there is no elegant solution to the large number of small states problem in the Northeast.

Second, I notice the color choice. The designer provides actual values on the visualization but also groups all state-average incomes into five categories. It's not clear how she/he determines the boundaries of these income brackets. There are many more dark blue states than there are light blue states in the map for men. Because women incomes are everywhere lower than men, the map at the bottom fits all states into two large buckets, plus Connecticut. Women incomes are lower than men but there is no need to break the data down by gender to convey this message.

Third, the use of two maps indicates that the designer does not care much about gender comparisons within each state. These comparisons are difficult to accomplish on the chart - one must involuntarily bob one's head up and down to make the comparisons. The head bobbing isn't even enough: then you must pull out your calculator and compute the ratio of women to men average. If the designer wants to highlight state-level comparisons, she/he could have plotted the gender ratio on a single map, like this:

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So far, I infer that the key questions are (a) the gender gap in aggregate (b) the variability of incomes within each gender, or the spatial clustering (c) the gender gap within each state.

(a) is better conveyed in more aggregate form. Goal (b) is defeated by the lack of clear clustering. (c) is not helped by the top-bottom split.

In making the above chart, I discover a pattern - that women fare better in the smaller states like Montana, Iowa, North & South Dakota. Meanwhile, the disparity in New York is of the same degree as Oklahoma and Wyoming.

 

 This chart tells readers a bit more about the underlying data, without having to print the entire dataset on the page.

 

 

 

A pretty good chart ruined by some naive analysis

The following chart showing wage gaps by gender among U.S. physicians was sent to me via Twitter:

The original chart was published by the Stat News website (link).

I am most curious about the source of the data. It apparently came from a website called Doximity, which collects data from physicians. Here is a link to the PR release related to this compensation dataset. However, the data is not freely available. There is a claim that this data come from self reports by 36,000 physicians.

I am not sure whether I trust this data. For example:

Do I believe that physicians in North Dakota earn the highest salaries on average in the nation? And not only that, they earn almost 30% more than the average physician in New York. Does the average physician in ND really earn over $400K a year? If you are wondering, the second highest salary number comes from South Dakota. And then Idaho.  Also, these high-salary states are correlated with the lowest gender wage gaps.

I suspect that sample size is an issue. They do not report sample size at the level of their analyses. They apparently published statistics at the level of MSAs. There are roughly 400 MSAs in the U.S. so at that level, on average, they have only 90 samples per MSA. When split by gender, the average sample size is less than 50. Then, they are comparing differences, so we should see the standard errors. And finally, they are making hundreds of such comparisons, for which some kind of multiple-comparisons correction is needed.

I am pretty sure some of you are doctors, or work in health care. Do those salary numbers make sense? Are you moving to North/South Dakota?

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Turning to the Visual corner of the Trifecta Checkup (link), I have a mixed verdict. The hover-over effect showing the precise values at either axes is a nice idea, well executed.

I don't see the point of drawing the circle inside a circle.  The wage gap is already on the vertical axis, and the redundant representation in dual circles adds nothing to it. Because of this construct, the size of the bubbles is now encoding the male average salary, taking attention away from the gender gap which is the point of the chart.

I also don't think the regional analysis (conveyed by the colors of the bubbles) is producing a story line.

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This is another instance of a dubious analysis in this "big data" era. The analyst makes no attempt to correct for self-reporting bias, and works as if the dataset is complete. There is no indication of any concern about sample sizes, after the analyst drills down to finer areas of the dataset. While there are other variables available, such as specialty, and other variables that can be merged in, such as income levels, all of which may explain at least a portion of the gender wage gap, no attempt has been made to incorporate other factors. We are stuck with a bivariate analysis that does not control for any other factors.

Last but not least, the analyst draws a bold conclusion from the overly simplistic analysis. Here, we are told: "If you want that big money, you can't be a woman." (link)

 

P.S. The Stat News article reports that the researchers at Doximity claimed that they controlled for "hours worked and other factors that might explain the wage gap." However, in Doximity's own report, there is no language confirming how they included the controls.

 

It's your fault when you use defaults

The following chart showed up on my Twitter feed last week. It's a cautionary tale for using software defaults.

 At first glance, the stacking of years in a bar chart makes little sense. This is particularly so when there appears not to be any interesting annual trend: the four segments seem to have roughly equal length almost everywhere.

This designer might be suffering from what I have called "loss aversion" (link). Loss aversion in data visualization is the fear of losing your data, which causes people to cling on to every little bit of data they have.

Several challenges of the chart come from the software defaults. The bars are ordered alphabetically, making it difficult to discern a trend. The horizontal axis labels are given in single dollars and units, and yet the intention of the designer is to use millions, as indicated in the chart titles.

The one horrifying feature of this chart is the 3D effect. The third dimension contains no information at all. In fact, it destroys information, as readers who use the vertical gridlines to estimate the lengths of the bars will be sadly misled. As shown below, readers must draw imaginary lines to figure out the horizontal values.

The Question of this chart is the distribution of book sales (revenues and units) across different genres. When the designer chose to stack the bars (i.e. sum the yearly data), he or she has decided that the details of specific years are not as important as the total - this is the right conclusion since the bar segments have similar measurement within each genre.

So let's pursue the revolution of averaging the data, plotting average yearly sales.

This chart shows that there are two major types of genres. In the education world, the unit prices of (text)books are very high while sales are relatively small by units but in aggregate, the dollar revenues are high. In the "adult" world, whether it's fiction or non-fiction, the unit price is low while the number of units is high, which results in similar total dollar revenues as the education genres.

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Simple lesson here: learn to hate software defaults