How to make a graded colourmap in Matplotlib with specified intervals - matplotlib

Question

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?

Answer 1

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()))
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)
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