QwtPlot is a composite widget consisting of a title label,
the canvas, the scales and a legend. Although all components 
should be exchangable some day, the current design isn´t ready for it.

In this situation event filtering is the mechanism to extend the behaviour
of the plot components. event_filter shows 3 examples how to use it:

1) CanvasPicker

The CanvasPicker implements a solution, how to move points on the canvas
with mouse and keyboard. 

2) ScalePicker

The ScalePicker translates the position of mouse clicks on the scales
and emits them as signals.

3) Plot: ColorBar, QSlider

The Plot class shows how to add widgets to the scales. In this example
there is no filter class. The derived plot widget filters its components.


Please note that CanvasPicker and ScalePicker are standalone classes
that could be connected with your QwtPlot as well. 

Uwe