Compare mode for LTM

You can now compare seasons / date ranges across or between
athletes on the LTM charts.

This is only shown on the stack chart as we need one chart
per data series - in a similar vein to the AllPlot chart.

There are some tidy ups left to do over the next few days;

- Data table needs updating to support compare mode
- Event markers need to be shown and in the right color
- PMC curve data is slow, needs some kind of optimisation
- The tooltip is missing and needs to be put back
- Ensure the new stack frame looks correct when using a
  dark plot background (or anything other than white)
- Consider how to handle zooming when there is only one
  data series and hence only one chart for compare