invTransform not working

[Uwe]

invTransform maps from widget to scale coordinates and a value of 3 is close to the beginning on an interval that is about [0, width() ]. So no surprise, that you are close to the lower bound.

Uwe

[ppp]

OK. And how can I get the right return value?
I need to find out what's the y value to a corresponding x value...

[Uwe]

OK. And how can I get the right return value?
I need to find out what's the y value to a corresponding x value...

By recalling what you have learned in school - this is basic maths completely unrelated to Qwt ( f.e see QLineF::pointAt() if you don't know how to calculate a point on a line ).
You could also have a look at the curvetracker example, what does something similar.

Uwe

[ppp]

I fixed my problem with the idea like in the curvetracker example without invTransform.
Thank you.