Qt Graphs 2D: Set series draw order/z-index

There is no way to configure the relative draw order or Z-index layering of data series within a Qt Graphs 2D chart.

We really need a way to set the relative Z-index ordering between overlapping data series, so that a specific data series is front, or in back, etc.

The current visual layering of series data is not configurable and execution produces arbitrary and inconsistent relative ordering.

Qt Charts has had this gap in its API for a decade and it's time to correct it with Qt Graphs.