Enable high-dpi support for plots

[akenmorris]

This PR enables higher resolution plots for high-dpi devices that do not use a 1.0 device pixel ratio. The issue is described well here:

https://stackoverflow.com/questions/42011410/qt-drawing-high-dpi-qpixmaps

The problem is that JKQtPlotter, like this example, is drawing to an offscreen QImage based on the "size" of the widget. However, a high-dpi display like a Mac Retina display will report half the actual size (e.g. 400x400 instead of 800x800) since these display units are given at the non-high-dpi scale.

Before:

After:

I can't say that I've done very extensive testing with these changes, but they have been suitable for my uses.

[sim186]

Hi! Is this already part of the latest release? If not how can I get your contribution?

[akenmorris]

Doesn't appear to have been merged. You can get it here:

https://github.com/akenmorris/JKQtPlotter/tree/high_dpi

[jkriege2]

Thanks for this fix ... and sorry it took so long (I've been unable to tend to this lib for the last two years due to personal reasons!)

[akenmorris]

No problem. Thanks for the amazing library! We are using it in ShapeWorks, e.g. :

http://sciinstitute.github.io/ShapeWorks/6.3/studio/getting-started-with-studio.html#metrics-panel