nonlinear colormap, matplotlib

Question

Are there any colormaps or is there a simple way to transform a matplotlib colormap to provide a much bigger color range near 0.5 and a smaller one at the extremes? I am cre

Answer 1

Your link provides quite a good solution for the colormap. I edited a bit, but it contained al the necessary. You need to pick some sensible levels for your nonlinear colormap. I used two ranges centered around the mean values, between +- 4 the standard deviation of your sample. by changing that to another number you obtain a different local gradient in the color around the two mean values.

For the colorbar, you

• either leave the colors nonlinearly spaced with linearly spaced labels
• you have linearly spaced colors with nonlinearly spaced labels.

The second allows greater resolution when looking at the data, looks nicer and is implemented below:

import numpy as np
import matplotlib.pyplot as plt

x = y = np.linspace(1, 10, 10)

t1mean, t2mean = 2, 9
sigma1, sigma2 = .3, .01
t1 = np.random.normal(t1mean, sigma1, 10)
t2 = np.random.normal(t2mean, sigma2, 10)

class nlcmap(object):
    def __init__(self, cmap, levels):
        self.cmap = cmap
        self.N = cmap.N
        self.monochrome = self.cmap.monochrome
        self.levels = np.asarray(levels, dtype='float64')
        self._x = self.levels
        self.levmax = self.levels.max()
        self.transformed_levels = np.linspace(0.0, self.levmax,
             len(self.levels))

    def __call__(self, xi, alpha=1.0, **kw):
        yi = np.interp(xi, self._x, self.transformed_levels)
        return self.cmap(yi / self.levmax, alpha)

tmax = max(t1.max(), t2.max())
#the choice of the levels depends on the data:
levels = np.concatenate((
    [0, tmax],
    np.linspace(t1mean - 4 * sigma1, t1mean + 4 * sigma1, 5),
    np.linspace(t2mean - 4 * sigma2, t2mean + 4 * sigma2, 5),
    ))

levels = levels[levels <= tmax]
levels.sort()

cmap_nonlin = nlcmap(plt.cm.jet, levels)

fig, (ax1, ax2) = plt.subplots(1, 2)

ax1.scatter(x, y, edgecolors=cmap_nonlin(t1), s=15, linewidths=4)
ax2.scatter(x, y, edgecolors=cmap_nonlin(t2), s=15, linewidths=4)

fig.subplots_adjust(left=.25)
cbar_ax = fig.add_axes([0.10, 0.15, 0.05, 0.7])

#for the colorbar we map the original colormap, not the nonlinear one:
sm = plt.cm.ScalarMappable(cmap=plt.cm.jet, 
                norm=plt.Normalize(vmin=0, vmax=tmax))
sm._A = []

cbar = fig.colorbar(sm, cax=cbar_ax)
#here we are relabel the linear colorbar ticks to match the nonlinear ticks
cbar.set_ticks(cmap_nonlin.transformed_levels)
cbar.set_ticklabels(["%.2f" % lev for lev in levels])

plt.show()

In the result, notice that the ticks of the colorbar are NOT equispaced: