Re: [Matplotlib-users] colormap from blue to transparent to red

[Scott S. <sco...@gm...>]

On 12 October 2010 14:57, Stefan Mauerberger
<ste...@mn...> wrote:
> I am having trouble with colormaps unsing pcolormesh. I would like to
> plot and colorise a seismic wave field above a map. Plotting works fine
> but I do not know how to bring transparency into colormaps. For negative
> values I want the coloration being blue then it should become
> transparent and the greatest value should be drawn red. I have tried a
> lot but without any success. As far as I can see, the keyarg alpha does
> not fit my needs at all.
>
> Do you have any suggestions for me?

You can't make the actual colormap contain transparent color entries,
but you can easily plot a masked array using a custom colormap. The
attached script based on
http://matplotlib.sourceforge.net/examples/pylab_examples/custom_cmap.html
should help you get what you want.

The important part of the script is to create a Numpy masked array to
exclude the regions you'd like to appear transparent:

cond = (-0.1 < Z) & (Z < 0.1)
Z_masked = np.ma.masked_where(cond, Z)

Cheers,
Scott