How to obtain values of a zoomed curve ?

[Hogwarts]

I need to obtain the values of a part of a curve, determined by a zoom.
I could achieve this by obtaining the indexes of the curve,determining the curve segment being shown, to use the sample function of QwtPlotSeriesItem and assuming that the curve values ​​were inserted in order, according to the passage of time. So far I could not do that. I know I could use the function QwtPlotZoomer::zoomRect(), and others but all efforts have been unsuccessful. I'm ussing Qwt 6.0
I know this has been asked before, but maybe in qwt 6.0 is easier to achieve this or there is another way.
Any help is welcome.

pd: sorry my english.