python - Matplotlib Legends not working

Question

Ever since upgrading matplotlib I get the following error whenever trying to create a legend:

/usr/lib/pymodules/python2.7/matplotlib/legend.py:610: UserWarning: Legend does not support [<matplotlib.lines.Line2D object at 0x3a30810>]
Use proxy artist instead.

http://matplotlib.sourceforge.net/users/legend_guide.html#using-proxy-artist

  warnings.warn("Legend does not support %s
Use proxy artist instead.

http://matplotlib.sourceforge.net/users/legend_guide.html#using-proxy-artist
" % (str(orig_handle),))
/usr/lib/pymodules/python2.7/matplotlib/legend.py:610: UserWarning: Legend does not support [<matplotlib.lines.Line2D object at 0x3a30990>]
Use proxy artist instead.

http://matplotlib.sourceforge.net/users/legend_guide.html#using-proxy-artist

  warnings.warn("Legend does not support %s
Use proxy artist instead.

http://matplotlib.sourceforge.net/users/legend_guide.html#using-proxy-artist
" % (str(orig_handle),))

This even occurs with a trivial script like this:

import matplotlib.pyplot as plt

a = [1,2,3]
b = [4,5,6]
c = [7,8,9]

plot1 = plt.plot(a,b)
plot2 = plt.plot(a,c)

plt.legend([plot1,plot2],["plot 1", "plot 2"])
plt.show()

I've found the link that the error points me towards pretty useless in diagnosing the source of the error.

Answer 1

You should add commas:

plot1, = plt.plot(a,b)
plot2, = plt.plot(a,c)

The reason you need the commas is because plt.plot() returns a tuple of line objects, no matter how many are actually created from the command. Without the comma, "plot1" and "plot2" are tuples instead of line objects, making the later call to plt.legend() fail.

The comma implicitly unpacks the results so that instead of a tuple, "plot1" and "plot2" automatically become the first objects within the tuple, i.e. the line objects you actually want.

http://matplotlib.sourceforge.net/users/legend_guide.html#adjusting-the-order-of-legend-items

line, = plot(x,sin(x)) what does comma stand for?