Grouped DP means plot should label axis and/or describe grouping

[mccalluc]

It doesn't describe what we're grouping by in the plot.

if groups:
    title = (
        f"DP means for grade, "
        f"assuming {contributions} contributions per individual"
    )
    plot_bars(grade_stats, error=0, cutoff=0, title=title)

Axis label might be better than title, though that might require more changes to plot_bars