One colorbar for multiple scatter plots

Question

I'm plotting multiple figures of the same variable on one plot using matplotlib library. I'm not looking for a colorbar for subplots, which is the dominant search material. I plot multiple scatters, but the colorbar is only set to the values of the last scatter I plot.

Here is the part of the code:

plt.scatter(x1, y1, c=z1,cmap='viridis_r',marker='s')
plt.scatter(x2, y2, c=z2,cmap='viridis_r',marker='o')
plt.scatter(x3, y3, c=z3,cmap='viridis_r',marker='^')
plt.colorbar().set_label('Wind speed',rotation=270)

Answer 1

It requires a bit of extra work:

• You have to get the minimum and maximum of the cs (the colorbar values)
• You have to clim each scatter plot

First the minimum and maximum:

zs = np.concatenate([z1, z2, z3], axis=0)
min_, max_ = zs.min(), zs.max()

Then the scatter plots with clim:

plt.scatter(x1, y1, c=z1,cmap='viridis_r',marker='s')
plt.clim(min_, max_)
plt.scatter(x2, y2, c=z2,cmap='viridis_r',marker='o')
plt.clim(min_, max_)
plt.scatter(x3, y3, c=z3,cmap='viridis_r',marker='^')
plt.clim(min_, max_)
plt.colorbar().set_label('Wind speed',rotation=270)

For a very simple dataset:

x1, x2, x3 = [1,2,3], [2,3,4], [3,4,5]
y1 = y2 = y3 = [1, 2, 3]
z1, z2, z3 = [1,2,3], [4,5,6], [7,8,9]

Answer 2

scatter has a norm argument. Using the same norm for all scatters ensures that the colorbar produced by any of the plots (hence also the last) is the same for all scatter plots.

The norm can be a Normalize instance, to which minimum and maximum value are set and which produces a linear scaling in between. Of course you can also use any other norm provided in matplotlib.colors like PowerNorm, LogNorm, etc.

mini, maxi = 0, 2  # or use different method to determine the minimum and maximum to use
norm = plt.Normalize(mini, maxi)
plt.scatter(x1, y1, c=z1,cmap='viridis_r',marker='s', norm=norm)
plt.scatter(x2, y2, c=z2,cmap='viridis_r',marker='o', norm=norm)
plt.scatter(x3, y3, c=z3,cmap='viridis_r',marker='^', norm=norm)
plt.colorbar().set_label('Wind speed',rotation=270)