Add sub single benchmark data point distribution density to single statistic charts

As suggested by jvahala in PLANNER-401:
Do a moving arithmetic average (or other sensible statistic) over data points for all subsingles, plot them on the same chart with the min/max bounds, add distribution shading (frequency of values in a given area on the chart) to visualize density

• JFreeChart probably doesn't support shading in this way
• If shading turns out to be a problem, an idea would be to print N lines, each representing a (1/N)th percentile in the subsingle data points