use legend titles in PX

Author: nicolaskruchten

doc/python/plotly-express.md

@@ -5,8 +5,8 @@ jupyter:
     text_representation:
       extension: .md
       format_name: markdown
-      format_version: "1.1"
-      jupytext_version: 1.1.1
+      format_version: '1.2'
+      jupytext_version: 1.3.1
   kernelspec:
     display_name: Python 3
     language: python
@@ -20,10 +20,9 @@ jupyter:
     name: python
     nbconvert_exporter: python
     pygments_lexer: ipython3
-    version: 3.7.3
+    version: 3.6.8
   plotly:
-    description:
-      Plotly Express is a terse, consistent, high-level API for rapid data
+    description: Plotly Express is a terse, consistent, high-level API for rapid data
       exploration and figure generation.
     display_as: file_settings
     language: python

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

@@ -1197,7 +1197,7 @@ def make_figure(args, constructor, trace_patch={}, layout_patch={}):
                     trace_name_labels[key] = str(val)
                 if m.variable == "animation_frame":
                     frame_name = val
-        trace_name = ", ".join(k + "=" + v for k, v in trace_name_labels.items())
+        trace_name = ", ".join(trace_name_labels.values())
         if frame_name not in trace_names_by_frame:
             trace_names_by_frame[frame_name] = set()
         trace_names = trace_names_by_frame[frame_name]
@@ -1346,7 +1346,7 @@ def make_figure(args, constructor, trace_patch={}, layout_patch={}):
     for v in ["title", "height", "width"]:
         if args[v]:
             layout_patch[v] = args[v]
-    layout_patch["legend"] = {"tracegroupgap": 0}
+    layout_patch["legend"] = dict(tracegroupgap=0, title=", ".join(trace_name_labels))
     if "title" not in layout_patch and args["template"].layout.margin.t is None:
         layout_patch["margin"] = {"t": 60}
     if (