matplotlib colorbar for scatter

Question

I'm working with data that has the data has 3 plotting parameters: x,y,c. How do you create a custom color value for a scatter plot?

Extending this example I'm trying to do:

import matplotlib import matplotlib.pyplot as plt cm = matplotlib.cm.get_cmap('RdYlBu') colors=[cm(1.*i/20) for i in range(20)] xy = range(20) plt.subplot(111) colorlist=[colors[x/2] for x in xy] #actually some other non-linear relationship plt.scatter(xy, xy, c=colorlist, s=35, vmin=0, vmax=20) plt.colorbar() plt.show() 

but the result is TypeError: You must first set_array for mappable

Answer 1

From the matplotlib docs on scatter 1:

cmap is only used if c is an array of floats

So colorlist needs to be a list of floats rather than a list of tuples as you have it now. plt.colorbar() wants a mappable object, like the CircleCollection that plt.scatter() returns. vmin and vmax can then control the limits of your colorbar. Things outside vmin/vmax get the colors of the endpoints.

How does this work for you?

import matplotlib.pyplot as plt cm = plt.cm.get_cmap('RdYlBu') xy = range(20) z = xy sc = plt.scatter(xy, xy, c=z, vmin=0, vmax=20, s=35, cmap=cm) plt.colorbar(sc) plt.show() 

Answer 2

Here is the OOP way of adding a colorbar:

fig, ax = plt.subplots() im = ax.scatter(x, y, c=c) fig.colorbar(im, ax=ax)