Center the colormap produced by partial_deriv_plot around 0

[cfe316]

Desired capability or behavior.

The check_partials tool coupled with partial_deriv_plot has been really useful in debugging some components and learning how to set up sparse partials.

One of the plots it produces is a map of the difference between J_fd and J_fwd. Right now it uses a diverging colormap, but it's not used quite properly. This sort of diverging colormap is designed for data centered around zero, where values of zero (meaning success in this application!) are gray, low values are blue, and high values are red.

Right now, the colormap has limits (probably set with a vmin and vmax) which are something like vmin=min(data), vmax=max(data). But, if positive errors are larger than negative errors, for example, then zero values are not gray, and the whole plot is tinted one way or the other.

One solution would be to precompute z = max(abs(min(data)), abs(max(data))) and set vmin=-z, vmax=z to force a centered color bar.

Associated POEM

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