Refine plots of numbers of infections

[swo]

          "Across the simulation generations" confused me

This is total number of infections, right?

The way it's written, I would have expected to see them colored by generation number -- which I actually like as an idea

For this PR, let's at least clarify that this is "total infections"?

Originally posted by @swo in #32 (comment)

[afmagee42]

There isn't a way to simply color this by generation (it's a histogram of the total/cumulative count, which can only be colored by things like "max non-extinct generation"), but we can make it so that users can control what they look at, which feels perhaps the better option?