#574 Control horizontal separation of outliers in boxplots

[Ethan Merritt]

If there are multiple outliers at the same y value the total width of the outlier point symbols can become unreasonably large. Possible improvements include
- apply the current jitter scale factor to boxplot outliers
- allow character type point symbols; these are not (I think!) affected by the point size setting, so one could use for example with boxplot pointtype '|' pointsize 0.1
- new property and keyword for boxplot style

[Ethan Merritt]

Queued for 6.0.2 "set jitter" applies to boxplot outliers