New issue 943: Derived fields not properly recognized in create profile https://bitbucket.org/yt_analysis/yt/issue/943/derived-fields-not-properly-r...
When creating a profile plot of a derived field, you can't access the derived field unless you originally defined it as ('gas', 'my_field') or ('index', 'my_field'). This is confusing, and I don't think it's supposed to work this way.
Example script that does not work: http://paste.yt-project.org/show/5226/
Script that fixes the problem: http://paste.yt-project.org/show/5228/