plot.arbutus legends

[mwpennell]

now that plot.arbutus uses plot instead of ggplot2, I can't figure out a nice way to place the legend for an arbitrary number of test statistics. this is not a high priority but it is sort of annoying.

[richfitz]

I guess this will be in the tutorial document, but a bit of code that gets this to the point where a legend is causing a problem would be useful. I have a couple of ideas.