Rotate matplotlib colourmap

Question

The ProPlot Python package adds additional features to the Matplotlib library, including colourmap manipulations. One feature that is particularly attractive to me is the ability to rotate/shift colourmaps. To give you an example:

import proplot as pplot
import matplotlib.pyplot as plt
import numpy as np

state = np.random.RandomState(51423)
data = state.rand(30, 30).cumsum(axis=1)

fig, axes = plt.subplots(ncols=3, figsize=(9, 4))
fig.patch.set_facecolor("white")

axes[0].pcolormesh(data, cmap="Blues")
axes[0].set_title("Blues")
axes[1].pcolormesh(data, cmap="Blues_r")
axes[1].set_title("Reversed Blues")
axes[2].pcolormesh(data, cmap="Blues_s")
axes[2].set_title("Rotated Blues")

plt.tight_layout()
plt.show()

In the third column, you see the 180° rotated version of Blues. Currently ProPlot suffers from a bug that doesn't allow the user to revert the plotting style to Matplotlib's default style, so I was wondering if there was an easy way to rotate a colourmap in Matplotlib without resorting to ProPlot. I always found cmap manipulations in Matplotlib a bit arcane, so any help would be much appreciated.

Answer 1

If what you are trying to do is shift the colormaps, this can be done (relatively) easily:

def shift_cmap(cmap, frac):
    """Shifts a colormap by a certain fraction.

    Keyword arguments:
    cmap -- the colormap to be shifted. Can be a colormap name or a Colormap object
    frac -- the fraction of the colorbar by which to shift (must be between 0 and 1)
    """
    N=256
    if isinstance(cmap, str):
        cmap = plt.get_cmap(cmap)
    n = cmap.name
    x = np.linspace(0,1,N)
    out = np.roll(x, int(N*frac))
    new_cmap = matplotlib.colors.LinearSegmentedColormap.from_list(f'{n}_s', cmap(out))
    return new_cmap

demonstration:

x = np.linspace(0,1,100)
x = np.vstack([x,x])

cmap1 = plt.get_cmap('Blues')
cmap2 = shift_cmap(cmap1, 0.25)

fig, (ax1, ax2) = plt.subplots(2,1)
ax1.imshow(x, aspect='auto', cmap=cmap1)
ax2.imshow(x, aspect='auto', cmap=cmap2)