Heatmap color gradient

[joda01]

Hello,

when I plot a heatmap the boxes are always ploted as gradient.
How can I switch that each box gets exact one color without gradient as shown in the examples.

Is:

Wanted:

Thanks a lot!

Best greetings

[DanteTrb]

Hi @joda01!

Most heatmaps (especially in Seaborn or matplotlib) use a continuous gradient by default.
But if you want each value to map to a discrete, solid color, without smooth interpolation, here’s a possible solution:

---

In matplotlib / seaborn (Python):

You need to:

• Use a ListedColormap with your fixed colors
• And define boundaries for the bins you want

import matplotlib.pyplot as plt
import numpy as np
import seaborn as sns
from matplotlib.colors import ListedColormap, BoundaryNorm

# Example data
data = np.array([[45, 60, 32],
                 [43, 54, 76],
                 [32, 94, 68],
                 [23, 95, 58]])

# Define bins and discrete colors
bounds = [20, 40, 60, 80, 100]  # Define value ranges
colors = ["#dbe9f6", "#a6cbe3", "#509dc5", "#1b70a6"]  # As in your image

cmap = ListedColormap(colors)
norm = BoundaryNorm(bounds, cmap.N)

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

# Optional: Add text labels
for i in range(data.shape[0]):
    for j in range(data.shape[1]):
        ax.text(j, i, f"{data[i, j]}", ha="center", va="center", color="black")

plt.colorbar(heatmap, ticks=bounds)
plt.show()

---

This disables interpolation entirely each value is mapped exactly to one color, just like your "Wanted" example.
If you're using seaborn.heatmap(), you can pass the same cmap and norm too!

Let me know if it works! 🥳