Zillow has posted a series of excellent diagrams which show relative changes in assorted housing markets, broken down by segment. Their diagrams are very clear and allow fairly quick access to a lot of good information. Please go explore there, and then come back here for my commentary.
By my count, there are eight different axes of information represented for each city or market:
- lower bound of each market
- upper bound of each market
- overall span of each market
- lower bound of each segment
- upper bound of each segment
- overall span of each segment
- direction of change for each segment
- magnitude of change for each segment
The first six of these axes are a natural byproduct of using the (horizontal) stacked bar chart format that they’ve chosen. Given how bad bar charts typically are (N.B. Excel), they did a great job with the shapes, colors, and generally minimalist approach. They’re really quite good.
To fully optimize the diagrams, I’d make a few minor changes.
Sorting the markets alphabetically makes it easy to find specific markets by name, but difficult to compare them. Regional groupings might be more useful. In an ideal world, the user could select a few markets by name and compare just those selected few.
The weakest point, in my opinion, is the ambiguity introduced by the arrows. I understand that they are meant to represent trends, not specific quantities. However, they can easily be made a little more clear.
Judging from the legend, the length of the shaft of the arrow represents the actual quantity, while the head encodes direction (redundantly with position and color). In some cases, such as our hometown of Seattle, the heads are ponderously large compared to the length of the shaft, making anything more than a cursory comparison impossible.
Removing the heads and using rectangular bars would make it easier to compare values at a glance. If we wanted to get more specific, the percent change per segment could be displayed within the segment oval, in red or green text.
While not quite as dramatic, I think these changes make the diagram more useful, without reducing their accessibility.
Finally, and this is a minor point, the legend states that “the top and bottom groups have been cut for better visualization.” This is entirely reasonable, but it made me curious about where they chose to trim those groupings. It would be nice to know what fraction of the actual number of homes are represented in the high- and low-end bars.