Python - Plotting colored grid based on values

Question

I have been searching here and on the net. I found somehow close questions/answers to what I want, but still couldn't reach to what I'm looking for.

I have an array of for example, 100 values. The values are in the range from 0 to 100. I want to plot this array as a grid, filling the squares according to the values in the array.

The solutions I found so far are like the followings:

Drawing grid pattern in matplotlib

and

custom matplotlib plot : chess board like table with colored cells

In the examples I mentioned, the ranges of the colors vary and are not fixed.

However, what I am wondering about, is whether I can set the ranges for specific values and colors. For example, if the values are between 10 and 20, let the color of the grid square be red. else if the values are between 20 and 30, let the color be blue. etc.

How this could be achieved in python?

Answer 1

You can create a ListedColormap for your custom colors and color BoundaryNorms to threshold the values.

import matplotlib.pyplot as plt
from matplotlib import colors
import numpy as np

data = np.random.rand(10, 10) * 20

# create discrete colormap
cmap = colors.ListedColormap(['red', 'blue'])
bounds = [0,10,20]
norm = colors.BoundaryNorm(bounds, cmap.N)

fig, ax = plt.subplots()
ax.imshow(data, cmap=cmap, norm=norm)

# draw gridlines
ax.grid(which='major', axis='both', linestyle='-', color='k', linewidth=2)
ax.set_xticks(np.arange(-.5, 10, 1));
ax.set_yticks(np.arange(-.5, 10, 1));

plt.show()

Resulting in;

For more, you can check this matplotlib example.

Answer 2

It depends on what units you need your colours to be in, but just a simple if statement should do the trick.

def find_colour(_val):
    # Colour value constants
    _colours = {"blue": [0.0, 0.0, 1.0],
                "green": [0.0, 1.0, 0.00],
                "yellow": [1.0, 1.0, 0.0],
                "red": [1.0, 0.0, 0.0]}

    # Map the value to a colour
    _colour = [0, 0, 0]
    if _val > 30:
        _colour = _colours["red"]
    elif _val > 20:
        _colour = _colours["blue"]
    elif _val > 10:
        _colour = _colours["green"]
    elif _val > 0:
        _colour = _colours["yellow"]

    return tuple(_colour)

And just convert that tuple to whatever units you need e.g. RGBA(..). You can then implement the methods it looks like you have already found to achieve the grid.