Output .png saving resolution and line_style

[martyniukvirtonomy]

Hello
Observing next problem and not sure how to handle it.
I want to save plot to png file with higher resolution 1200x800 and specific line style.
matplot::figure(true)->size(1280, 720);
plot(x,y, "--")
then saving with
matplot::gcf()->save("filename.png", "png");
I get resolution I need, but dashed lines are rendered as solid.

If I use save without format like:
matplot::gcf()->save("filename.png");
I have dashed lines but default resolution (640,480).

Resolution issue was mentioned here #166
Could you advice how to handle it? Thank you in advance