Can't plot legend of different kind of lines

[zxdawn]

Description

If multiple kinds of line are plotted on the figure, the legend() doesn't work.

Steps to reproduce

import proplot as pplt
import numpy as np
state = np.random.RandomState(51423)
data = (state.rand(10, 5) - 0.5).cumsum(axis=0)

fig, axs = pplt.subplots()

data = state.rand(20)
l1 = axs.plot(data, label='l1')
l2 = axs.plot(data*2, label='l2')
l3 = axs.axhline(0.5, c='green', label='l3')
axs.legend([l1, l2, l3], loc='r')

Expected behavior
proplot 0.6.4 works well:

Actual behavior

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/tmp/ipykernel_31462/545939102.py in <module>
     11 l3 = axs.axhline(0.5, c='green', label='l3')
     12 # axs.legend([l1, l2], loc='r')
---> 13 axs.legend([l1, l2, l3], loc='r')

~/new/miniconda3/envs/pyresample_min/lib/python3.8/site-packages/proplot/ui.py in _iterator(*args, **kwargs)
    780                 result = []
    781                 for func in objs:
--> 782                     result.append(func(*args, **kwargs))
    783                 if len(self) == 1:
    784                     return result[0]

~/new/miniconda3/envs/pyresample_min/lib/python3.8/site-packages/proplot/axes/base.py in legend(self, handles, labels, loc, width, space, queue, **kwargs)
   1869         if loc in ('left', 'right', 'top', 'bottom'):
   1870             ax = self.panel_axes(loc, width=width, space=space, filled=True)
-> 1871             obj = ax.legend(handles, labels, loc='fill', **kwargs)
   1872             self._add_colorbar_legend(loc, obj, legend=True)  # add to *this* axes
   1873             return obj

~/new/miniconda3/envs/pyresample_min/lib/python3.8/site-packages/proplot/axes/base.py in legend(self, handles, labels, loc, width, space, queue, **kwargs)
   1895 
   1896         # Generate legend
-> 1897         obj = wrap.legend_extras(self, handles, labels, loc=loc, **kwargs)
   1898         self._add_colorbar_legend(loc, obj, legend=True)  # possibly replace another
   1899         return obj

~/new/miniconda3/envs/pyresample_min/lib/python3.8/site-packages/proplot/axes/plot.py in legend_extras(self, handles, labels, loc, ncol, ncols, center, order, label, title, fontsize, fontweight, fontcolor, color, marker, lw, linewidth, dashes, linestyle, markersize, frameon, frame, **kwargs)
   4344     if list_of_lists:
   4345         if any(not np.iterable(_) for _ in handles):
-> 4346             raise ValueError(f'Invalid handles={handles!r}.')
   4347         if not labels:
   4348             labels = [None] * len(handles)

ValueError: Invalid handles=[[<matplotlib.lines.Line2D object at 0x7ffacb87cfd0>], [<matplotlib.lines.Line2D object at 0x7ffacb87cca0>], <matplotlib.lines.Line2D object at 0x7ffa42564d90>].

Proplot version

matplotlib: 3.3.4
proplot: 0.7.0

[lukelbd]

Matplotlib's plot always returns lists of handles (and proplot does not change this behavior). In your code it looks like you are putting together handles and singleton lists of handles within lists. I guess v0.6.4 allowed this but 0.7 reverted back to matplotlib's behavior.

For now you can use:

axs.legend([*l1, *l2, l3], loc='r')  # option A
axs.legend(l1 + l2 + [l3])  # option B
# etc.

In a PR I'm working on legend input is more flexible -- you will be able to use mixed list/non-list input, among other things.

[lukelbd]

This is fixed on master now as part of the larger legend() overhaul. Tested your example and I get three legend entries.