PR curve step re-selection broken

[wchargin]

Unfortunately, it looks like #3524 breaks PR curves: the initial render
is fine, but when you change the step slider the plot is just replaced
by a series that only shows the last data point. Repro video:
pr_curve_redraw_broken.webm.zip

Occurs since 2020-04-23 nightly and on master; can be fixed on master by
reverting 27d0023.

[wchargin]

I’m going to roll back for now. Feel free to roll forward with fix.

[stephanwlee]

#3552 has fixed the issue and #3571 fixes the underlying issue.