xychart: Pivot position for ctrl+scroll zooming is not adjusted correctly

[ghost]

When ctrl+scrolling to zoom in and out, the mouse position can sometimes be used as a "pivot" for the zoom operation (either because the selection is not in the current visible range, or if the shift key is used to force it).

Right now this pivot position is not determined correctly in XYChart widgets. It would need to be adjusted with regards to the chart area. This is not as bad as with selection, where it's immediately obvious the position is offset. The same adjustment as the position used for selection should be used.