This chart (shown right), published by Zillow in a report on housing in 2012, looks quite standard, apparently avoiding the worst of Excel defaults.
In real estate, it’s all about location. In dataviz, it’s all about details.
What are some details that I caught my eye on this chart?
Readers have to get over the hurdle that “negative equity” is the same as “underwater homes.” This is not readily understood unless one reads the surrounding text. For example, the first row for the U.S. average proclaims that 31% of U.S. homes are “underwater” and among these underwater homes, 10% of the mortgages are delinquent. The former is concerned with the valuation of the property while the latter deals with payments or lack thereof.
According to the legend, the blue segments stand for the proportions of underwater homes in different metro areas but it’s not quite true – the blue part represents underwater but not delinquent mortgages while the red and blue combined represents all underwater mortgages. This is a common problem in stacked bar charts.
The metro areas are in alphabetical order by city, which means an opportunity is missed to help readers discern patterns. Patterns related to city-name alphabets is not of interest to most (except certain econometrics journal editors). Try arranging by region, or by decreasing level of negative equity, or some other meaningful variable.
The designer tried to do something clever with the horizontal axis labels and I don't think it succeeds. To see what is going on, read the note below the chart. The trick is to let readers look at the number of underwater and delinquent mortgages in two ways, as a proportion of underwater mortgages (through the white data labels) and as a proportion of all mortgages (through the axis labels). That's a mess, sorry to say.
Finally, I like the horizontal axis to extend to 100% because underlying the proportions shown in blue and on the horizontal axis is the population of all mortgages.
Perhaps a shock to many readers. The task of showing underwater delinquent mortgages simultaneously as a proportion of underwater mortgages and as a proportion of all mortgages is solved using .... pie charts.
I just created a couple of examples here:
The deep orange sector can be compared to the entire circle, or to the larger orange sector. Readers usually don't have a problem with pies with only three slices.