Your code assigns widget coordinates as plot coordinates ...

The easiest way to disable autoscaling ( what is not what you mean by autoscaling ! ) is to assign a scale. But you are looking for QwtPlotRescaler. Check the navigation example - the second combo box should do what you need.

Note that it is not always possible to have a fixed aspect ratio for the plot canvas. The problem here is a ping-pong effect between the adjustments of the scales ( according to the size of the canvas ) and the space that is needed for the resulting tick labels, that might modify the size that is left for the canvas.

Uwe