Fill area with color depending on third value array

[Uwe]

I recommend to use one of the development branches > Qwt 6.1, where you find the FilterPointsAggressive flag, that will reduce the number of points to be painted to a reasonable number.

Then overload QwtPlotCurve::fillCurve, the polygon parameter contains the points to be painted already being translated in widget coordinates. When using the QwtScaleMap
you can always translate between widget and plot coordinates.

Have a look at the implementation of QwtPlotCurve::fillCurve - the code is only truth, when trying to understand how certain things work.

HTH,
Uwe