See RT-14249 for a similar issue about custom background regression.

I would be good to be able to set a custom foreground in XYChart on top of the plot area. 1.x used to provide a similar facility to allow to set a custom background, that is a node that was always showing behind the plot area. Something similar would be needed but for displaying nodes ontop of the plot area instead. This could be used to display markers on the X and Y axis and plot area and possibly some controls that could be manipulated by the user too.

Additionnaly : the API needs to be expanded from 1.x to allow acces to the dimensions of the plot area. In 1.x it was possible to get the rectangular dimension of the plot are by projecting the upper and lower bounds of the X and Y axis of the chart but this felt more like a hack of some sort.