Fill area with color depending on third value array

[Vasya]

Uwe, Thank for the answer,

Yes, first of all I don't understand how to make gradient steps related to the QWT axes coordinate system.
Actually I don't fully understand what input parameters of void QGradient::setColorAt(qreal position, const QColor &color) means beacause qreal position is a single number and I have two numbers (one for x- and other for y-axis).

So in my case I'm trying to implement the second plot of https://www.mathworks.com/matlabcent...screenshot.png
So for any (x0, y0) point I need to set color as bright as z0 value is.

I tried:

QLinearGradient linearGrad(QPointF(0, 1), QPointF(3, 1));
    linearGrad.setCoordinateMode(QGradient::StretchToDeviceMode);
    linearGrad.setColorAt(0, Qt::black);
    linearGrad.setColorAt(0.2, Qt::white);
    curve->setBrush(linearGrad);
    curve->setBaseline(1);
 
    // ???????? ????? ?? ????? ????????? ??????
    QPolygonF points;
 
           points << QPointF( 0.0, 0.0 ) // ?????????? x, y
        << QPointF( 0.5, 1.5 ) << QPointF( 1.0, 1.5 )
        << QPointF( 1.5, 1.0 ) << QPointF( 2.0, 0.5 )
        << QPointF(2.5, 0.5) << QPointF(3.0, 1.0);

QLinearGradient linearGrad(QPointF(0, 1), QPointF(3, 1));
    linearGrad.setCoordinateMode(QGradient::StretchToDeviceMode);
    linearGrad.setColorAt(0, Qt::black);
    linearGrad.setColorAt(0.2, Qt::white);
    curve->setBrush(linearGrad);
    curve->setBaseline(1);

    // ???????? ????? ?? ????? ????????? ??????
    QPolygonF points;

           points << QPointF( 0.0, 0.0 ) // ?????????? x, y
        << QPointF( 0.5, 1.5 ) << QPointF( 1.0, 1.5 )
        << QPointF( 1.5, 1.0 ) << QPointF( 2.0, 0.5 )
        << QPointF(2.5, 0.5) << QPointF(3.0, 1.0);

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And I get the image:
qtasdf.jpg
But I can't understand how points are related to the axes coordinate system?
And the next step would be how to code color with z-value array for each point? I suspect it would be done with void QGradient::setColorAt(qreal position, const QColor &color) and the algorithm how to transform z-value to the color is well known in Internet and I think will find the answer for this.