errorbar "caps" do not obey zorder

[kecnry]

this is a known bug within MPL. There are some possible workarounds (search, there are too many links to provide here) that we may be able to use, such as forcing all passed zorders to be > 2.

See here: https://stackoverflow.com/questions/14003572/how-to-force-errorbars-to-render-last-with-matplotlib and links to MPL issues within