3D plots are incorrect if axes are not regular

[darafferty]

3D plots show incorrectly if an axis is irregularly sampled. For example, in a plot of amplitude solutions over freq vs time, if the frequency axis is not regular, it's mislabeled/distorted -- shown below are the correct frequencies:

and the 3D plot (which doesn’t show the proper frequency spacing above ~168 MHz):

I think this issue is due to imshow() assuming a regular grid, if so, we might fix it by interpolating to a regular grid before plotting (or at least warn the user that it might be distorted).