how to get the (x, f(x)) values under the cursor?

[Vincenzo]

Hi,

I have a QwtPlot. I'd like to get the value of point under the mouse cursor. Yet I can recieve the coordinates:

QwtPicker *picker = new QwtPicker(this);
    picker->setTrackerMode(QwtPicker::AlwaysOn);

QwtPicker *picker = new QwtPicker(this);
    picker->setTrackerMode(QwtPicker::AlwaysOn);

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,but how can I get the value of x and f(x), also how should I convert coordinates to values, and vice versa?

Furthermore, how could I insert text to the QwtPlot, because I want put the values next to the points.

Please, insert short example code, since I'm absolute beginner...
Thank's in advance.