Thread: [Matplotlib-users] 2D array plot picking explicit values to colour and legend

Dear matplotlib-users,

I have a spatial data set that has coded values for each cell, which are 
limited to just a few numbers, ie -8888, 0, 100, and 9999. I would like 
to display this data with a plot similar to pcolor, but I don't want a 
colorbar, I want a legend showing the colors for each code and an 
explanation for what each code represents. I would like to be able to 
choose a subset of the codes as well, for example just plotting the 0 
and 100 codes and ignoring the -8888 and 9999 codes.

I have seen a few similar attempts that used BoundaryNorm, but I don't 
want to show a range of values I just want to set colors for a few 
explicit values. Those examples also had a colorbar and, as I said, I 
would prefer a legend.

Can anyone offer any tips?

Thanks

Mat

-- 
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.

On 04/09/2012 02:22 AM, Mathew Topper wrote:
> Dear matplotlib-users,
>
> I have a spatial data set that has coded values for each cell, which are
> limited to just a few numbers, ie -8888, 0, 100, and 9999. I would like
> to display this data with a plot similar to pcolor, but I don't want a
> colorbar, I want a legend showing the colors for each code and an
> explanation for what each code represents. I would like to be able to
> choose a subset of the codes as well, for example just plotting the 0
> and 100 codes and ignoring the -8888 and 9999 codes.
>
> I have seen a few similar attempts that used BoundaryNorm, but I don't
> want to show a range of values I just want to set colors for a few
> explicit values. Those examples also had a colorbar and, as I said, I
> would prefer a legend.
>
> Can anyone offer any tips?

For the plot itself you can use pcolor if your data are on a 
quadrilateral grid, or a PathCollection or PolyCollection otherwise. 
For the legend, you can use proxy artists:

http://matplotlib.sourceforge.net/users/legend_guide.html#using-proxy-artist

Eric

>
> Thanks
>
> Mat
>

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.

On 04/09/2012 08:17 AM, Mathew Topper wrote:
> Hi Eric, thanks for the tip about the legend.
>
> Regarding the data, assuming i am using pcolor, am I right in thinking
> that using Boundarynorm would be the best way to control the colors for
> each code?

Mat,

I think BoundaryNorm is overkill and/or awkward for your case.  It 
sounds like you don't have ordinary values, but rather a set of labels 
that happen to be integers.  I would use a ListedColormap and then use 
sequential integers as the C values to index directly into the colormap:

C = np.array([[0,1,2],[2,0,1]])
import matplotlib.colors as mcolors
cmap = mcolors.ListedColormap(['r', 'g', 'lightgray'])
pcolor(C, cmap=cmap, norm=mcolors.NoNorm())

Of course you would need to map your sequence of numbers (-8888, 0, ...) 
to a sequence of integers starting at zero.

The key point is that the NoNorm() instance leaves your original C 
values alone, and since they are integers, they are then used directly 
as indices.

You could also make your own mcolors.Normalize subclass which would 
process your labels and return either a float in the 0-1 range, or an 
integer for direct indexing.

When you need only a very few colors, the ListedColormap with direct 
indexing is nice because it allows you to specify those colors using any 
valid color specification method.


Eric



>
> Thanks
>
> Mat
>
> On 04/09/2012 06:26 PM, Eric Firing wrote:
>> On 04/09/2012 02:22 AM, Mathew Topper wrote:
>>> Dear matplotlib-users,
>>>
>>> I have a spatial data set that has coded values for each cell, which are
>>> limited to just a few numbers, ie -8888, 0, 100, and 9999. I would like
>>> to display this data with a plot similar to pcolor, but I don't want a
>>> colorbar, I want a legend showing the colors for each code and an
>>> explanation for what each code represents. I would like to be able to
>>> choose a subset of the codes as well, for example just plotting the 0
>>> and 100 codes and ignoring the -8888 and 9999 codes.
>>>
>>> I have seen a few similar attempts that used BoundaryNorm, but I don't
>>> want to show a range of values I just want to set colors for a few
>>> explicit values. Those examples also had a colorbar and, as I said, I
>>> would prefer a legend.
>>>
>>> Can anyone offer any tips?
>> For the plot itself you can use pcolor if your data are on a
>> quadrilateral grid, or a PathCollection or PolyCollection otherwise.
>> For the legend, you can use proxy artists:
>>
>> http://matplotlib.sourceforge.net/users/legend_guide.html#using-proxy-artist
>>
>> Eric
>>
>>> Thanks
>>>
>>> Mat
>>>
>>
>> ------------------------------------------------------------------------------
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>
> --
> Dr. Mathew Topper
> Institute for Energy Systems
> School of Engineering
> The University of Edinburgh
> Faraday Building
> The King’s Buildings
> Edinburgh EH9 3JL
> Tel: +44 (0)131 650 5570
> School fax: +44 (0)131 650 6554
> mat...@ed... <mailto:mat...@ed...>
> http://www.see.ed.ac.uk <http://www.see.ed.ac.uk/>
>
>
>
> The University of Edinburgh is a charitable body, registered in
> Scotland, with registration number SC005336.
>
>
>
> ------------------------------------------------------------------------------
> For Developers, A Lot Can Happen In A Second.
> Boundary is the first to Know...and Tell You.
> Monitor Your Applications in Ultra-Fine Resolution. Try it FREE!
> http://p.sf.net/sfu/Boundary-d2dvs2
>
>
>
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