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Rick Wicklin

An advantage of the heatmap (as presented) is that it shows missing data better. The line plots in the small multiples are "connecting through missing," which results in an incorrect plot for Teenage Pregnancy (the datum for 2003 is not shown). If the line plots are redesigned to not connect through missing, then for some data the display could have many disconnected lines, which would probably look strange to most viewers. For more on using heatmaps to visualize time series, see Peng (2008), JSS: http://www.jstatsoft.org/v25/c01

Kaiser

Rick: Point taken, but is it more important to show off the missing values or is it more important to show the trends?

Rick Wicklin

Trends, for these data. Line plots show the trends better. The heatmap has other nice attributes, such as the ability to compare down a column to see, for example, that 2004 was a "bad year" across most categories. I think the heatmap also can handle 100 variables better than the small multiples.

Maybe we could combine the plots? Move the category labels to the left of the heatmap and add a sparkline to the right of each row?

Kaiser

For me, the heatmap is great in its compactness and it's very useful when looking for sharp differences, say outliers. But it isn't so good for understanding trends or mild differences because the human eye is much better at judging shifts in levels than in judging shifts in colors. I'll read the Peng paper to see if there's something there.
If you really want to expose the missing values, add an asterisk on the time axis for each missing value.

Chris

Using red dots and green dots to indicate significance is also a nightmare for the colorblind. The original chart makes my eyes sore :(

Stef

Reminds me very much of the poster we did a couple of years back. Although I really liked the design, I have to admit that deciphering the trends of the different variables was always a little bit difficult, although perhaps somewhat clearer than on the example given above. I think that otherwise, only the small multiples would be a solution for a clearer understanding.

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