Plot Area Custom Gradient Fill

I'm trying to format the plot area of a X-Y scatter chart in a three colour gradient (Green, Yellow & Red).

The trick is, I would like the green portion to be centred in a user defined place on the plot with the yellow radiating from it, and the red radiating from the yellow. Ideally I would like to define some stops for the green as a user defined shape, with stops for the yellow outside the green stops (like the % stops you get in a vertical or angled gradient, but instead of the stops being simple straight lines or concentric circles from the middle, they would be user defined shapes).

I can do the normal types of gradient, but have no idea how to customise them in this way.