How to make a graded colourmap in Matplotlib with specified intervals

sobmortin354 0
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I am trying to make a custom colourmap in matplotlib but can't quite get it right. What I want is a specific colour for specific values, but for there to be a smooth gradation between them. So something along the lines of this:

colourA = {
    0.01: "#51158d"
    0.25: "#5c2489"
    0.5:  "#693584"
    1:    "#9d9933"
    3:    "#e3dc56"
}

I've seen the method LinearSegmentedColormap.from_list() which I can pass all the hex codes into, but how can I assign specific colours to specific values so that the steps are not equally distributed?

colormap matplotlib
1 Answers

You don't have to use the from_list classmethod, that just exists for convenience, assuming a commonly used equal spacing between them.

You can convert your colors to the cdict required for the LinearSegmentedColormap. Starting with what you already have:

import matplotlib as mpl

colors = {
    0.01: "#51158d",
    0.25: "#5c2489",
    0.5:  "#693584",
    1:    "#9d9933",
    3:    "#e3dc56",
}
my_cmap = mpl.colors.LinearSegmentedColormap.from_list("my_cmap", list(colors.values()))

enter image description here

You basically need to do two things; normalize your range between 0 and 1, and convert the hex format to rgb.

For example:

from collections import defaultdict

cdict = defaultdict(list)

vmin = min(colors.keys())
vmax = max(colors.keys())

for k,v in colors.items():
    
    k_norm = (k - vmin) / (vmax - vmin)
    r,g,b  = mpl.colors.to_rgb(v)
    
    cdict["red"].append((k_norm, r, r))
    cdict["green"].append((k_norm, g, g))
    cdict["blue"].append((k_norm, b, b))
    
my_cmap = mpl.colors.LinearSegmentedColormap("my_cmap", cdict)

enter image description here

Note that the cdict requires the x-values to increase monotonically, which is the case with this example. Otherwise first sort the colors in the dictionary.

When using this to plot, you can/should use a norm to scale it between whatever values you want, for example:

norm = mpl.colors.Normalize(vmin=0.01, vmax=3)

Details about the format of the cdict can be found in the docstring:
https://matplotlib.org/stable/api/_as_gen/matplotlib.colors.LinearSegmentedColormap.html

Rutger Kassies0
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