I think that one of the most common problems when trying to visualize data is overcrowding. This can happen whenever one tries to display too many data categories, too many data points, or both. Below is a map from the National Weather Service that displays the current national weather warnings and advisories.
The main problem with this map is that it tries to cram way too many types of warnings and advisories into a single view. There are a total of 30 categories, and all are coded by color. Eventually it becomes almost impossible to distinguish between them. For example, does the coast of Hawaii have a Coastal Flood Warning, High Surf Warning, or both? Further investigation revels that it was a High Surf Waring. There are even more problems when one considers the interaction that a user might have with the map. For one, clicking on a state and using the drop down menu to select one produce completely different results.
The current map and others can be found here
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Wow. If one ever wanted an example of why you can’t encode large numbers of categories with color, here it is. The large number of colors on the map are more or less impossible to match to the listed conditions, even with close inspection of the legend.
I think they could, to an extent, improve on this encoding somewhat. If, instead of choosing colors purely on category, they encoded several classes of weather conditions with color families and severity with color intensity (winter storm watch -> winter storm warning -> blizzard warning in deeper shades of blue), things might be a little easier to understand. Of course, you still have weather conditions that don’t fit well into a continuum, and quickly run out of color families for weather conditions.