Fine-tune the logic for which users are color-coded and listed in the
graphs and which aren't. Rather than a fixed percent, users are now
sorted by greatest number of submissions in a month and the top N are
selected over two passes (the first with exemptions for users active in
the pie chart months, the second without if the first wasn't enough to
whittle it down to N).
N is currently set to 18 since that seems to be just enough to fill up
the bar chart legend.