New quantity in splash

[abbasebriastro]

When the run was over, I wanted to add a new quantity to the splash list while plotting, I know that I have to do this from the options section.
My question is:
To plot the Ω (Ω=v/r) as function the core radius "r", how should I do it? Since v itself also has components v_x and v_y!
May anyone help me that?