Fix Issue 2966 px.pie has no legend title

Author: lucafs

packages/python/plotly/plotly/express/_core.py

@@ -2146,6 +2146,9 @@ def make_figure(args, constructor, trace_patch=None, layout_patch=None):
     layout_patch["legend"] = dict(tracegroupgap=0)
     if trace_name_labels:
         layout_patch["legend"]["title_text"] = ", ".join(trace_name_labels)
+    elif('showlegend' in trace_patch.keys()):
+        if(trace_patch['showlegend'] and constructor == go.Pie):
+            layout_patch["legend"]["title_text"] = args['names']
     if args["title"]:
         layout_patch["title_text"] = args["title"]
     elif args["template"].layout.margin.t is None:

packages/python/plotly/plotly/tests/test_core/test_graph_objs/test_pie.py

@@ -0,0 +1,24 @@
+import plotly.express as px
+import plotly.graph_objects as go
+from numpy.testing import assert_equal
+import numpy as np
+import pandas as pd
+import pytest
+
+
+def _compare_legend_to_column_name(name_column, name_from_fig):
+    """Compare legend title name to the name given to it 
+    in "names" value to see if is equal to it and not None.
+    """
+    assert_equal(name_from_fig, name_column)
+    assert name_from_fig
+
+
+def test_pie_like_px():
+    # Pie
+    data_name = 'col1'
+    names_name = 'col2'
+    df = pd.DataFrame(data={data_name: [3, 2, 1], names_name: [1, 2, 4]})
+    fig = px.pie(df, values=data_name, names=names_name,
+                 title='Population of European continent')
+    _compare_legend_to_column_name(names_name, fig.layout.legend.title.text)