How to add a colorbar for iteratively added patches?

Question

I am iteratively adding rectangular patches to a plot that are coloured according to a sensor reading. How would I go about adding a colorbar to my plot based on these values? Given that these patches are not stored in an array, I expect I need to define a colorbar spanning a range of values independently of what is being plotted. How would I do this?

Answer 1

You can create a colorbar from a ScalarMappable, given a colormap and a norm. The norm needs the minimum and maximum of the values and projects input values to the range 0,1.

The code below uses a generator to simulate reading the measurements. A norm is created for input values between 0 and 20. relim and autoscale_view make sure the ax's limits are recalculated for each new rectangle.

import matplotlib.pyplot as plt
from matplotlib.patches import Rectangle
import numpy as np

def get_next_data():
    for i in range(20):
        value = np.random.uniform(0, 20)
        x = np.random.uniform(0, 30)
        y = np.random.uniform(0, 10)
        yield x, y, value

fig, ax = plt.subplots()
minv, maxv = 0, 20
cmap = plt.cm.coolwarm
norm = plt.Normalize(minv, maxv)
sm = plt.cm.ScalarMappable(cmap=cmap, norm=norm)
sm.set_array([])
plt.colorbar(sm, ax=ax, label='Values')
for x, y, val in get_next_data():
    patch = Rectangle((x, y), 1, 1, color=cmap(norm(val)))
    ax.add_patch(patch)
    x += 1
    ax.relim()
    ax.autoscale_view()
    plt.pause(1)
plt.show()