This allows snapshoting the motion highlight separate from the rest of the
layer so that we can have better render tree diffs. Otherwise we risk
having to snapshot a large dataset from the chart layer itself rather than
just the hightlight areas that changed.
The goal here is to be able to connect data series such as
SysprofTimeSeries or SysprofXYSeries to various chart layers. Those
chart layers get stacked in a SysprofChart.
Some glue code will be necessary to go from frames into data series but
we can experiment with how to automate that later on.
