Thanks! I almost get it. One more question: are coordinates on canvas widget and coordinates that shown at QwtScaleDraw not the same (coordinates that shown at QwtScaleDraw are converted coordinates of canvas widget)? I mean if, for example, our Y axis has 100 as min value and 200 for max value and X axis has the same limits, then is point [0;0] of canvas widget (top-left corner) the same as point [200, 100] of our graph (or it is more complicated)?