Test/hammer out bug(s) in boxplots

[fedarko]

I haven't been able to replicate this, but I think that occasionally the boxplot whiskers can get "disconnected" when you switch to a boxplot. In my experience (only seen this a few times—only once with the current implementation of boxplots that I developed on better-boxplots), this goes away if you double-click the boxplot checkbox.

I think there's a decent chance that this is just a bug with the boxplot mark in Vega-Lite. May be related to vega/vega-lite/issues/4925 and, therefore, vega/vega-lite/issues/4524 (the problem image shown in #4925 looks super similar to what I've seen, albeit without using temporal data).

In any case: I'm gonna just keep this issue open for the time being, and if this issue hasn't popped up again in say a few months I'll close this. (And in the mean time—or at least until I get them carefully tested—I'm gonna mark boxplots as "experimental.")