Matplotlib Legend for Scatter with custom colours


I'm a bit of newbie at this and am trying to create a scatter chart with custom bubble sizes and colours. The chart displays fine but how do I get a legend saying what the colours refer to. This is as far as I've got:

inc = []
out = []
bal = []
col = []


inc = (30000,20000,70000)
out = (80000,30000,40000)
bal = (12000,10000,6000)
col = (1,2,3)
leg = ('proj1','proj2','proj3')

ax.scatter(inc, out, s=bal, c=col)
ax.axis([0, 100000, 0, 100000])

ax.set_xlabel('income', fontsize=20)
ax.set_ylabel('Expenditure', fontsize=20)
ax.set_title('Project FInancial Positions %s' % dt)

This thread was helpful, but couldn't get it to solve my problem: Matplotlib: Legend not displayed properly

5/23/2017 12:30:26 PM

Accepted Answer

Maybe this example is helpful.

In general, the items in the legend are related with some kind of plotted object. The scatter function/method treats all circles as a single object, see:

print type(ax.scatter(...))

Thus the solution is to create multiple objects. Hence, calling scatter multiple times.

Unfortunately, newer version of matplotlib seem not to use a rectangle in the legend. Thus the legend will contain very large circles, since you increased the size of your scatter plot objects.

The legend function as a markerscale keyword argument to control the size of legend markers, but it seems to be broken.


The Legend guide recommends using Proxy Artist in similar cases. The Color API explains valid fc values.

p1 = Rectangle((0, 0), 1, 1, fc="b")
p2 = Rectangle((0, 0), 1, 1, fc="g")
p3 = Rectangle((0, 0), 1, 1, fc="r")
legend((p1, p2, p3), ('proj1','proj2','proj3'))

To get the colors used previously in a plot, use the above example like:

pl1, = plot(x1, y1, '.', alpha=0.1, label='plot1')
pl2, = plot(x2, y2, '.', alpha=0.1, label='plot2')
p1 = Rectangle((0, 0), 1, 1, fc=pl1.get_color())
p2 = Rectangle((0, 0), 1, 1, fc=pl2.get_color())
legend((p1, p2), (pl1.get_label(), pl2.get_label()), loc='best')

This example will make a plot like:

Matplotlib with custom legend

12/21/2012 3:00:59 PM

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