Well. there is not so much to explain: YourTransform::transform is used for mapping a plot coordinate to a widget coordinate and YourTransform::invTransform for the other way round.
The formula how these methods are used is given in the docs: http://qwt.sourceforge.net/class_qwt...m.html#details.

The scaleengine example shows a couple of transformations f.e one that uses a different formula for a specific interval ( TransformPos in mainwindow.cpp ). Instead of the path, where pow is used you have to add your A/x code.

Uwe