Transparency of overlaid contour fill without effect on underlying isocontours

[jfrieben]

I have tried to overlay two contour plots by setting the transparency of the first lower one to something like 0.2 and that of the upper one to about 0.8 for both of fill and line colours. While the fill colours of the lower plot appear washed out as expected, the transparency value of the upper plot has no effect at all.
In the attached sample picture, the colour of a number of isocontours in the lower plot has been set to white and the transparency to 1.0. Moreover, the alpha values of fill and contour colours of the upper plot have been set to 1.0. As can be seen from the figure, the colour of the lower contour lines is bold white even in the region where they should be hidden by the second overlaid plot.
This issue affects all matplotlib versions from 0.99.1.2 when I tried this first up to the current release 1.3.1.

[tacaswell]

Could you provide code for a minimal example that exercises this bug? That makes reproducing, and hence fixing, it much easier.

[efiring]

The zorder might be causing this. Lines have a higher default zorder than patches, so they are drawn on top.

[jfrieben]

Thanks a lot, efiring! Adding the option `zorder=...' with suitable values does indeed lead to a correct shading of the contour lines.

[mdboom]

In the interest of keeping our issue tracker clean, I'll close this, but @jfrieben if you think there's a way we could make this work better from matplotlib's end, feel free to continue to discuss -- we can always reopen.