When there are many signals in the design, the size of the majority of the histogram bins is large. However, some bins may still contain a small number of items (as shown in the image in the Solution section below). The problem is that it becomes very difficult to select the bins with a small number of items relative to that of larger bins. Being able to select the bin is important so that the list of paths comprising that bin can be seen.
While I can make the Y axis logarithmic when creating the histogram, I would like a way to improve usability of an already drawn histogram.
In the Vivado 2012.4 and PlanAhead 14.4 tools, all bars are at least 3 pixels high, so all non-zero count bars will be visible.
In general, linear scale is best if the count is 1, and log scale works better if the count is greater than 1.