Markers aren't cleared when DevMarkerChart changes DataSource

[stewienj]

Markers aren't cleared when DevMarkerChart changes DataSource. So we end up with ghost markers.

There seems to be a todo comment here:

DynamicDataDisplayReloaded/src/DynamicDataDisplay.Markers/DevMarkerChart.cs

Line 50 in fadded8

// todo clear

Not sure why this hasn't been implemented.