shiftcmap.py

   1 import numpy as np
   2 import matplotlib
   3 import matplotlib.pyplot as plt
   4
   5 def shiftedColorMap(cmap, start=0, midpoint=0.5, stop=1.0, name='shiftedcmap'):
   6                 '''
   7                 Function to offset the "center" of a colormap. Useful for
   8                 data with a negative min and positive max and you want the
   9                 middle of the colormap's dynamic range to be at zero.
  10
  11                 Input
  12                 -----
  13                         cmap : The matplotlib colormap to be altered
  14                         start : Offset from lowest point in the colormap's range.
  15                                         Defaults to 0.0 (no lower offset). Should be between
  16                                         0.0 and `midpoint`.
  17                         midpoint : The new center of the colormap. Defaults to
  18                                         0.5 (no shift). Should be between 0.0 and 1.0. In
  19                                         general, this should be  1 - vmax / (vmax + abs(vmin))
  20                                         For example if your data range from -15.0 to +5.0 and
  21                                         you want the center of the colormap at 0.0, `midpoint`
  22                                         should be set to  1 - 5/(5 + 15)) or 0.75
  23                         stop : Offset from highest point in the colormap's range.
  24                                         Defaults to 1.0 (no upper offset). Should be between
  25                                         `midpoint` and 1.0.
  26                 '''
  27                 cdict = {
  28                                 'red': [],
  29                                 'green': [],
  30                                 'blue': [],
  31                                 'alpha': []
  32                 }
  33
  34                 # regular index to compute the colors
  35                 reg_index = np.linspace(start, stop, 257)
  36
  37                 # shifted index to match the data
  38                 shift_index = np.hstack([
  39                                 np.linspace(0.0, midpoint, 128, endpoint=False),
  40                                 np.linspace(midpoint, 1.0, 129, endpoint=True)
  41                 ])
  42
  43                 for ri, si in zip(reg_index, shift_index):
  44                                 r, g, b, a = cmap(ri)
  45
  46                                 cdict['red'].append((si, r, r))
  47                                 cdict['green'].append((si, g, g))
  48                                 cdict['blue'].append((si, b, b))
  49                                 cdict['alpha'].append((si, a, a))
  50
  51                 newcmap = matplotlib.colors.LinearSegmentedColormap(name, cdict)
  52                 plt.register_cmap(cmap=newcmap)
  53
  54                 return newcmap