Re: [Matplotlib-users] possible kludge for masked value colors in contourf?

[Eric F. <ef...@ha...>]

James Boyle wrote:
> As far as I can tell, you cannot specify the color that contourf uses  
> for masked values.
> The examples enclosed with matplotlib ( 0.98.3) specifically comment  
> that this is the case.
> It appears that contourf just does not plot anything where the masked  
> values occur and the background ( usually white ) is what is seen.
> This is very useful but ....
> 
> I would like to have a bit more choice in how the areas of masked  
> data are represented.
> I came up with the following hack which seems to work, but being far  
> from expert I wanted to know if anyone wished to help or give advice.
> Is this reasonable or what ?

Jim,

It strikes me as the right way to do what you want; it is not a kludge 
or a hack at all.  Contourf generates patches for unmasked regions, not 
for the masked region, so it is indeed up to you to set the background 
as you have done.

I see that there is also an Axes.set_axis_bgcolor method, which I would 
expect to be preferred; but it is strangely named.  Should it not be 
just Axes.set_bg or Axes.set_bgcolor?  It corresponds to the axisbg 
kwarg in the Axes initializer, perhaps another unfortunate choice of 
names.

(Contourf has a longstanding bug in handling internal masked regions; I 
hope you don't run into it.  Both Mike D and I have tried to figure out 
how to fix that bug, but so far without success.)

> 
> In the code below data is a masked array with some areas masked.
> The result is that the missing areas are now black rather than white  
> ( the default background, I guess)
> 
> fig = pylab.figure()
> ax = fig.add_subplot(111)
> ax.axesPatch.set_facecolor('k')
> cs = ax.contourf(data)
> pylab.savefig('example')
> 
> I am aware that pcolor , pcolormesh and using cm.set_bad will work  
> but I also wanted the option for contourf - it produces nice looking  
> plots.

I have not checked just now, but as I recall, pcolor acts like contourf 
with respect to masked regions--it plots nothing there, leaving the 
background showing through.

Eric