Exclude colour from a colourbar

Community matplotlib-users
1

Hi,

I am setting a colourbar where I explictly set the tick intervals. However I
would like to exclude the colour black from this colourbar. The reason for
this is I would like to overplot some contour lines in black, so would like
to make them stand out.

An example...where I would like to exclude black as the bottom colour of the
colourbar, i.e. use the next level of gray.

import numpy as np
from mpl_toolkits.basemap import Basemap
import matplotlib.pyplot as plt
import matplotlib.colors as colors
import matplotlib as mpl

data = np.random.randint(0,50,100*100).reshape(100, 100)
m = Basemap(llcrnrlon=1.5, llcrnrlat=10.5, urcrnrlon=3.5, urcrnrlat=13.5,
            resolution='c', projection='cyl')
fig = plt.figure(figsize=(8, 6))
ax = fig.add_axes([0.1, 0.1, 0.6, 0.7])
m.ax = ax

colourmap = plt.cm.Greys_r
ticks = [0, 10, 20, 30, 40, 50]
norm = colors.BoundaryNorm(ticks, colourmap.N)
im = m.imshow(data, colourmap, norm=norm, interpolation='nearest')
pos = ax.get_position()
l, b, w, h = pos.bounds
cax = plt.axes([l+w+0.045, b, 0.05, h])
cbar = mpl.colorbar.ColorbarBase(cax, cmap=colourmap, norm=norm,
ticks=ticks)
plt.show()

Many thanks,

Martin

···

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2

So far I have tried this...

colourmap = cm.get_cmap("Greys_r")
colourmap = colourmap._segmentdata

# exclude 0, 0, 0...i.e. black from dictionary
bvals = colourmap['blue'][1:]
gvals = colourmap['green'][1:]
rvals = colourmap['red'][1:]
colourmap = { "blue":bvals, "green":gvals, "red":rvals }
colourmap = cm.set_cmap(colourmap)

But it doesn't seem to like the set_cmap command

colourmap = cm.set_cmap(colourmap)
AttributeError: 'module' object has no attribute 'set_cmap'

So I guess that isn't the way, nor I suspect is that all that elegant.

Martin

mdekauwe wrote:

···

Hi,

I am setting a colourbar where I explictly set the tick intervals. However
I would like to exclude the colour black from this colourbar. The reason
for this is I would like to overplot some contour lines in black, so would
like to make them stand out.

An example...where I would like to exclude black as the bottom colour of
the colourbar, i.e. use the next level of gray.

import numpy as np
from mpl_toolkits.basemap import Basemap
import matplotlib.pyplot as plt
import matplotlib.colors as colors
import matplotlib as mpl

data = np.random.randint(0,50,100*100).reshape(100, 100)
m = Basemap(llcrnrlon=1.5, llcrnrlat=10.5, urcrnrlon=3.5, urcrnrlat=13.5,
            resolution='c', projection='cyl')
fig = plt.figure(figsize=(8, 6))
ax = fig.add_axes([0.1, 0.1, 0.6, 0.7])
m.ax = ax

colourmap = plt.cm.Greys_r
ticks = [0, 10, 20, 30, 40, 50]
norm = colors.BoundaryNorm(ticks, colourmap.N)
im = m.imshow(data, colourmap, norm=norm, interpolation='nearest')
pos = ax.get_position()
l, b, w, h = pos.bounds
cax = plt.axes([l+w+0.045, b, 0.05, h])
cbar = mpl.colorbar.ColorbarBase(cax, cmap=colourmap, norm=norm,
ticks=ticks)
plt.show()

Many thanks,

Martin

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3

The solution incase anyone has a similar issue...

data = np.random.randint(0,50,100*100).reshape(100, 100)
m = Basemap(llcrnrlon=1.5, llcrnrlat=10.5, urcrnrlon=3.5, urcrnrlat=13.5,
                    resolution='c', projection='cyl')
fig = plt.figure(figsize=(8, 6))
ax = fig.add_axes([0.1, 0.1, 0.6, 0.7])
m.ax = ax
ticks = [0, 10, 20, 30, 40, 50]
colours = cm.Greys_r(np.linspace(0.3, 1.0, 6))
colourmap = colors.ListedColormap(colours)
norm = colors.BoundaryNorm(ticks, colourmap.N)
im = m.imshow(data, cmap=colourmap, norm=norm, interpolation='nearest')
pos = ax.get_position()
l, b, w, h = pos.bounds
cax = plt.axes([l+w+0.045, b, 0.05, h])
cbar = mpl.colorbar.ColorbarBase(cax, cmap=colourmap, norm=norm,
ticks=ticks)
plt.show()

···

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