refactor pandas df.append to pd.concat in _core

Author: BjoernLudwigPTB

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

@@ -1634,7 +1634,9 @@ def aggfunc_continuous(x):
         df_tree["parent"] = df_tree["parent"].str.rstrip("/")
         if cols:
             df_tree[cols] = dfg[cols]
-        df_all_trees = df_all_trees.append(df_tree, ignore_index=True)
+        df_all_trees_old = df_all_trees.append(df_tree, ignore_index=True)
+        df_all_trees = pd.concat([df_all_trees, df_tree], ignore_index=True)
+        assert df_all_trees.equals(df_all_trees_old)
 
     # we want to make sure than (?) is the first color of the sequence
     if args["color"] and discrete_color: