Improve multiple semantic legend titles

Currently the legends for lineplot (#1285) and scatterplot (#1436) don’t indicate what variable each of the levels represent. I punted on this but it needs a solution. One option is to cram everything into the legend title but that’s not ideal. I think doing something with legend entries that have invisible handles and and then bumping the legend text so it looks centered would work:

f, ax = plt.subplots()

h1 = ax.scatter([], [], s=0, label="Title 1")
h2 = ax.scatter([], [], label="level 1")
h3 = ax.scatter([], [], s=0, label="Title 2")
h4 = ax.scatter([], [], label="level 2")

legend = ax.legend()
for t in legend.get_texts()[::2]:
    xfm = transforms.offset_copy(t.get_transform(), ax.figure, x=-10, units="points")
    t.set_transform(xfm)

But getting this to be really robust is going to be tricky.

[mwaskom]

Why not?

f, ax = plt.subplots()
ax.scatter(1, 1, visible=False)

[CRiddler]

Ah I just saw your comment as I was coming to post this. I think checking on the legend artist visibility wouldn’t be too hard and be a more appropriate proxy than alpha because we can set the visibility of the handle afterwards. What I was just thinking is that instead of even adding the subtitles to the Axes at all (right now, we’re calling ax.scatter([], [], label="title 1") solely for the label) we don’t really want anything to do with the scatter what if we had the user supply the ordering in a wrapper around ax.legend()? Sorry for throwing a billion ideas at you!

import seaborn as sns
import matplotlib
import matplotlib.pyplot as plt

def subtitle_legend(ax, legend_format):
    new_handles = []
    
    handles, labels = ax.get_legend_handles_labels()
    label_dict = dict(zip(labels, handles))
    
    #Means 2 labels were the same
    if len(label_dict) != len(labels):
        raise ValueError("Can not have repeated levels in labels!")
    
    for subtitle, level_order in legend_format.items():
        #Roll a blank handle to add in the subtitle
        blank_handle = matplotlib.patches.Patch(visible=False, label=subtitle)
        new_handles.append(blank_handle)
        
        for level in level_order:
            handle = label_dict[level]
            new_handles.append(handle)

    #Labels are populated from handle.get_label() when we only supply handles as an arg
    legend = ax.legend(handles=new_handles)

    #Turn off DrawingArea visibility to left justify the text if it contains a subtitle
    for draw_area in legend.findobj(matplotlib.offsetbox.DrawingArea):
        for handle in draw_area.get_children():
            if handle.get_label() in legend_format:
                draw_area.set_visible(False)

    return legend


def seaborn_scatter():
    sns.set()
    
    tips = sns.load_dataset('tips')
    ax = sns.scatterplot(x="total_bill", y="tip",
                         hue="sex", style="smoker", size="size",
                         data=tips)

    #nice and explicit call to reorder the legend
    legend_format = {'Sex (hue)': ['Male', 'Female'],
                     'Group Size (size)': ['0', '2', '4', '6'],
                     'Smoker (shape)': ['Yes', 'No']
                    }

    # Use this instead of `ax.legend()`
    #   this returns the legend as well for further tweaking
    #   can also set it up to take kwargs to pass onto `ax.legend` call
    subtitle_legend(ax, legend_format=legend_format)

    plt.show()

seaborn_scatter()