Recently, I noticed that yt lacked a feature I needed to analyze a protostellar disk simulation. For this work, I'd like to be able to plot the average azimuthal and radial velocity as a function of radius. Since my disk is not aligned with the grid, it's a bit of a pain to do this while referring only to the grid-aligned coordinates.
To fix that situation, I took it upon myself to add new fields that project the velocity and magnetic field onto a cylindrical or spherical coordinate system with an arbitrary orientation.
If you're on the development branch of yt, the next time you run 'yt update', you'll have several new derived fields available:
cyl_TangentialVelocity, cyl_RadialVelocity, BPoloidal, BPoroidal, BPadial
These fields supplement the already available TangentialVelocity and RadialVelocity, which refer to a spherical coordinate system.
The new fields allow me to write a script like this: http://paste.yt-project.org/show/2786/
To make this work, I've written a framework for converting coordinates and vector fields from cartesian to cylindrical and spherical coordinates. If you have some other vector field that you'd like to look at in a different coordinate system, this framework should be exactly what you need!
If you'd like more details, let me know and I'll do my best to help.