Python Matplotlib Colormap

Question

I use the colormap \"jet\" to plot my graphics. But, I would like to have the lower values in white color and this colormap goes from blue to red colors. I also don\'t want

Answer 1

There is another way to do the same without defining a new colobar. You can use the cmap.set_under method that defines the color to be used for all values below a given threshold. The threshold is defined during the pcolormesh :

from matplotlib.pyplot import *
import numpy as np

mycmap = cm.get_cmap('jet')
mycmap.set_under('w')

pcolor(np.random.rand(10,10), cmap=mycmap, vmin=.1)
colorbar()
show()

Answer 2

There's pretty much this example on the colormaps page. Digging through the matplotlib source on my machine, jet is defined as (in _cm.py):

_jet_data =   {'red':   ((0., 0, 0), (0.35, 0, 0), (0.66, 1, 1), (0.89,1, 1),
                         (1, 0.5, 0.5)),
               'green': ((0., 0, 0), (0.125,0, 0), (0.375,1, 1), (0.64,1, 1),
                         (0.91,0,0), (1, 0, 0)),
               'blue':  ((0., 0.5, 0.5), (0.11, 1, 1), (0.34, 1, 1), (0.65,0, 0),
                         (1, 0, 0))}

Answer 3

Probably there should be an easiest solution, but the way I figured out is by creating your own matplotlib.colors.LinearSegmentedColormap, based on the "jet" one.

(The lowest level of your colormap is defined in the first line of each tuple of red, green, and blue, so that's where you start editting. I add one extra tuple to have a clearly white spot in lower part. ...for each color, in the first element of the tuple you indicate the position in your colorbar (from 0 to 1), and in the second and third the color itself).

from matplotlib.pyplot import *
import matplotlib
import numpy as np

cdict = {'red': ((0., 1, 1),
                 (0.05, 1, 1),
                 (0.11, 0, 0),
                 (0.66, 1, 1),
                 (0.89, 1, 1),
                 (1, 0.5, 0.5)),
         'green': ((0., 1, 1),
                   (0.05, 1, 1),
                   (0.11, 0, 0),
                   (0.375, 1, 1),
                   (0.64, 1, 1),
                   (0.91, 0, 0),
                   (1, 0, 0)),
         'blue': ((0., 1, 1),
                  (0.05, 1, 1),
                  (0.11, 1, 1),
                  (0.34, 1, 1),
                  (0.65, 0, 0),
                  (1, 0, 0))}

my_cmap = matplotlib.colors.LinearSegmentedColormap('my_colormap',cdict,256)

pcolor(np.random.rand(10,10),cmap=my_cmap)
colorbar()
show()

You'll get the following: