How to get the point's coordinate in the qwtplotcurve?

[changanminglike]

first I create a qwtplotcurve using some points,after that I want to get the point coordinate in the qwtplotcurve.any help will be appreciate.thanks!

[Spitfire]

Which point you want to get?
Certain point on the screen?
Mouse position on the curve?
Y coordinate given some X?

[changanminglike]

Thanks,I want to get the Point's x coordinate and Y coordinate.and I get the result just now.the code are as follow:
double x=(qwtPlotCurve->data()).x(pointId);
double y=(qwtPlotCurve->data()).y(pointId);