Polygon handles not passing to legend

I am trying to plot some polygons and lines from geopandas dataframe and add a legend. However, when plotting the polygons, the labels don’t get passed to the legend handles.

So for example

    fig,ax=plt.subplots()
    lineshp.plot(ax=ax,color='k', linewidth=3,label='line')
    polyshp.plot(ax=ax,color='r',label='poly')
    ax.get_legend_handles_labels()

It returns: ([<matplotlib.collections.LineCollection at 0x116681d0>], [‘line’]) and does not include the polygon handles.

Anyone has an idea of how I could fix this? This seems to be a new issue for me.

[skytruine]

I’m using geopandas 0.8.1, and I also faced this problem. Here is my solution

# load China polygon
    rgn = gpd.read_file(
        'https://geo.datav.aliyun.com/areas_v2/bound/100000_full.json')
    # Administrative divisions at provincial level in China (polygon)
    china = rgn[rgn.level.isna() == False]
    # South China Sea Nine-dash line (polygon)
    nine_lines = rgn[rgn.level.isna()]

    # Drawing features
    fig, ax = plt.subplots(figsize=(8, 8))
    ax = china.plot(ax=ax,
                    facecolor='grey',
                    edgecolor='white',
                    linestyle='--',
                    alpha=0.8,
                    label='Administrative divisions')
    ax = nine_lines.plot(ax=ax,
                         edgecolor='grey',
                         linewidth=3,
                         alpha=0.4,
                         label='South China Sea Nine-dash line')

    # Error in normal writing: No handles with labels found to put in legend.
    # ax.legend(title='Legend', loc='lower left', ncol=1, shadow=True)

    # Alternative solution for "Polygon handles not passing to legend"
    import matplotlib.patches as mpatches
    import matplotlib.pyplot as plt
    pmark = mpatches.Patch(facecolor='grey',
                           edgecolor='white',
                           linestyle='--',
                           alpha=0.8,
                           label='Administrative divisions')
    lmark = mlines.Line2D(
        [],
        [],
        color='grey',
        linewidth=3,
        alpha=0.4,
        label='South China Sea Nine-dash line')
    handles, _ = ax.get_legend_handles_labels()
    ax.legend(
        handles=[
            *handles,
            lmark,
            pmark],
        title="Legend",
        loc='lower left',
        ncol=1,
        shadow=True)
    
    # show figure
    plt.rcParams['legend.title_fontsize'] = 10
    plt.show()

Although this approach can fulfill my requirements, it’s really cumbersome, and I really hope that geopandas can fix it.

[jorisvandenbossche]

OK, this is a ‘limitation’ of matplotlib. They only support automatically creating legend handlers for some collections, so for points and lines they do, but not for PatchCollections (polygons). So we could in principle, if a label is passed, create ourselves a legend handle, and pass that to matplotlib.

See https://matplotlib.org/2.0.0/users/legend_guide.html