Give the possibility to supply a custom label generator for pie charts

[ggiambo]

See #698.

Demo of PieChart02 with labels showing percentage:

Before

After

[mccartney]

Looks good to me. This seems reasonable feature to add.

Thanks for the contribution.

[timmolter]

Thanks for putting in the effort to change the implementation from your previous PR. Can you change the demo to be in the same format at in

XChart/xchart-demo/src/main/java/org/knowm/xchart/demo/charts/date/DateChart09.java

Line 67 in a9f75d4

DateTimeFormatter cursorXFormatter = DateTimeFormatter.ofPattern("LLL dd");

, i.e.

            x -> startTime.plusDays(x.longValue()).format(cursorXFormatter)

, without needing to define chart.getSeriesMap().values()?

[ggiambo]

Hi @timmolter
I'm not sure I full understand your request. I guess you want to avoid the call to Chart#getSeriesMap, that is marked for removal, am I right?
In commit 44f6a18 I tried to gather the total without accessing to the whole series map.