Purpose:
#2148

Note that this PR doesn't yet add this for BarSeries, I'm writing this between classes so I'll add it later.

The other question is whether this should be customizable, is drawing the text above negative bars always wrong (i.e. this is a bug fix), or should the user be able to choose that behaviour (i.e. this is a new feature). If it's the latter, I think we should preserve the previous behaviour of ShowValuesAboveBars and add ShowValuesAtTipOfBar or something similar.

double[] values = { 27.3, 23.1, -21.2, 16.1, 6.4, 19.2, 18.7, 17.3, 20.3, 13.1 };

var bar = plt.AddBar(values);
bar.ShowValuesAboveBars = true;
bar.Orientation = Orientation.Horizontal;