"""

hexbin is an axes method or pyplot function that is essentially a

pcolor of a 2-D histogram with hexagonal cells.

"""

import numpy as np

import matplotlib.pyplot as plt

import matplotlib.mlab as mlab

delta = 0.025

x = y = np.arange(-3.0, 3.0, delta)

X, Y = np.meshgrid(x, y)

Z1 = mlab.bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)

Z2 = mlab.bivariate_normal(X, Y, 1.5, 0.5, 1, 1)

Z = Z2 - Z1 # difference of Gaussians

x = X.ravel()

y = Y.ravel()

z = Z.ravel()

if 1:

# make some points 20 times more common than others, but same mean

xcond = (-1 < x) & (x < 1)

ycond = (-2 < y) & (y < 0)

cond = xcond & ycond

xnew = x[cond]

ynew = y[cond]

znew = z[cond]

for i in range(20):

x = np.hstack((x, xnew))

y = np.hstack((y, ynew))

z = np.hstack((z, znew))

xmin = x.min()

xmax = x.max()

ymin = y.min()

ymax = y.max()

gridsize = 30

plt.subplot(211)

plt.hexbin(x, y, C=z, gridsize=gridsize, marginals=True, cmap=plt.cm.RdBu,

vmax=abs(z).max(), vmin=-abs(z).max())

plt.axis([xmin, xmax, ymin, ymax])

cb = plt.colorbar()

cb.set_label('mean value')

plt.subplot(212)

plt.hexbin(x, y, gridsize=gridsize, cmap=plt.cm.Blues_r)

plt.axis([xmin, xmax, ymin, ymax])

cb = plt.colorbar()

cb.set_label('N observations')

plt.show()