Re-add the "value_font" setting used by pie charts

This was mistakenly dropped in f3b33e9.  Bar charts use "values_font",
and so had no issues.  Without the proper font, wide characters were
displayed incorrectly, and the automatically chosen font was
additionally small and unreadable.
Merge branch '4.2/pie-chart-font' into 4.2.0-releng