No handles with labels found to put in legend

Question

I'm trying to create a parallelogram in PyPlot. I'm not up to drawing the parallelogram--first I'm putting in the vector arrows--using the following code:

fig = plt.figure() ax = fig.add_subplot(111) ax.spines['left'].set_position('zero') ax.spines['right'].set_color('none') ax.spines['bottom'].set_position('zero') ax.spines['top'].set_color('none') plt.axis([-5,5,-5,5]) ax.xaxis.set_ticks_position('bottom') ax.yaxis.set_ticks_position('left') plt.grid() plt.arrow(0,0, 3,1, head_width=0.2, color='r', length_includes_head=True, label='u') plt.arrow(0,0, 1,3, head_width=0.2, color='r', length_includes_head=True, label='v') plt.arrow(0,0, 4,4, head_width=0.2, color='r', length_includes_head=True, label='u+v') plt.legend() 

This returns the following error:

No handles with labels found to put in legend. 

I'm not sure why, because, based on the documentation for plt.arrow(), label is an acceptable kwarg, and plt.legend() should ostensibly be reading that. The rest of the figure draws fine; it's just missing the legend.

Answer 1

It might be late but for anyone with the same issue the solution is using the method legend() for the corresponding ax not as for plt

fig = plt.figure() ax = fig.add_subplot(111) ax.spines['left'].set_position('zero') ax.spines['right'].set_color('none') ax.spines['bottom'].set_position('zero') ax.spines['top'].set_color('none') plt.axis([-5,5,-5,5]) ax.xaxis.set_ticks_position('bottom') ax.yaxis.set_ticks_position('left') plt.grid() plt.arrow(0,0, 3,1, head_width=0.2, color='r', length_includes_head=True, label='u') plt.arrow(0,0, 1,3, head_width=0.2, color='r', length_includes_head=True, label='v') plt.arrow(0,0, 4,4, head_width=0.2, color='r', length_includes_head=True, label='u+v') ax.legend() 

Answer 2

You can explicitly define the elements in the legend.

For full control of which artists have a legend entry, it is possible to pass an iterable of legend artists followed by an iterable of legend labels respectively. Reference

Example:

arr1 = plt.arrow(0,0, 3,1, head_width=0.2, color='r', length_includes_head=True) arr2 = plt.arrow(0,0, 1,3, head_width=0.2, color='g', length_includes_head=True) arr3 = plt.arrow(0,0, 4,4, head_width=0.2, color='b', length_includes_head=True)  plt.xlim(0,5) plt.ylim(0,5)  plt.legend([arr1, arr2, arr3], ['u','v','u+v'])