QwtPlotSpectrogram and Color dpending on the axis

Hello,

I have a QwtPlotSpectrogram. It is rendering as a Waterfall. So if I set the colormap, colors depend on the value of the Z Axis.

How can I change it. If I want to have color depending on the value of the X axis for example.

Thanks.

solved : juste need to change the value of the Z axis and return the good value with the model