no, you can't (without implementing that by yourself). a curve is basically a series of points. QwtPlotCurve can give you the nearest point to a specified coordinate.

If you want exactly the y-value for any x-value, you have to find the x coordinates x_left and x_right of the nearest points where your x0 is in the middle. the curve is a straight line between these two points. now you need the y-values y_left and y_right of these two points. Then you can calculate the y0 using some triangulation.

hth,
Felix