I had a quick look at your changes and IMHO it seems to me like you are working around issues that have to do with rounding errors because of an integer based implementation. Your patches somehow fix these problems by adding some margins here changing a factor there. This might be a solution when using a formula with QwtTextLabel, but when scaling comes into the game ( f.e. a formula on the canvas painted with QwtPlotRenderer to PDF ) your approach will not be enough.
IMO a more promising approach would be to modify the renderer being completely floating point based ?
Uwe
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