Setting the midpoint of a matplotlib colormap

default_norm.png

my_norm.png

SetMidpoint.py

import matplotlib as mpl
import matplotlib.pyplot as plt
import numpy as np

class MidpointNormalize(mpl.colors.Normalize):
    """
    class to help renormalize the color scale
    """
    def __init__(self, vmin=None, vmax=None, midpoint=None, clip=False):
        self.midpoint = midpoint
        mpl.colors.Normalize.__init__(self, vmin, vmax, clip)

    def __call__(self, value, clip=None):
        # I'm ignoring masked values and all kinds of edge cases to make a
        # simple example...
        x, y = [self.vmin, self.midpoint, self.vmax], [0, 0.5, 1]
        return np.ma.masked_array(np.interp(value, x, y))

# generate data for test plots
x_values = np.random.normal( 0, 1, 10000)
y_values = np.random.normal( 0, 1, 10000)

# plot with default midpoint
fig, ax = plt.subplots()
default_norm = ax.hexbin(x_values, y_values,
                         cmap=plt.cm.seismic)
default_norm = ax.hexbin(x_values, y_values,
                         cmap=plt.cm.seismic,
                         gridsize = (25,25))
fig.colorbar(default_norm)
plt.savefig('default_norm.png')
plt.close()

# plot with custom midpoint
fig, ax = plt.subplots()
norm = MidpointNormalize(midpoint = 20)
my_norm = ax.hexbin(x_values, y_values,
                    cmap=plt.cm.seismic,
                    norm = norm,
                    gridsize = (25,25))
fig.colorbar(my_norm)
plt.savefig('my_norm.png')
plt.close()