You can't simply draw to a widget - this is not how Qt works. On most platforms you can't event paint outside paint events at all. You also need to take care of the plot -> widget transformation, that is changing on different type of layout events or when zooming in/out. But having a class around a couple of lines of render code ( this is what your is plot item about) is nothing to be afraid of.
QwtPlotMarker has a mode for drawing such a line - but I simply forgot to add a similar feature to QwtPolarMarker.
Uwe
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