Change default colour for spatial selection in chart widget

[ClarkLangridge]

We've noticed that for certain backgrounds, the colour that shows up for doing a spatial selection in the Chart widget is really hard to see.

This has lead to some of our users getting really frustrated because they can't tell if they have started drawing the polygon or not.

It would be really helpful to be able to change that colour, either in the initial setup of the widget or allow the end-user to change it, so that it is easier to see against whatever background is currently visible.