special case for wide-var=columns

Author: nicolaskruchten

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

@@ -1239,7 +1239,13 @@ def build_dataframe(args, constructor):
         elif wide_x != wide_y:
             wide_mode = True
             args["wide_variable"] = args["y"] if wide_y else args["x"]
-            var_name = "variable"
+            if df_provided and args["wide_variable"] is df_input.columns:
+                var_name = df_input.columns.name
+                args["wide_variable"] = list(args["wide_variable"])
+            if var_name in [None, "value", "index"] or (
+                df_provided and var_name in df_input
+            ):
+                var_name = "variable"
             if constructor == go.Histogram:
                 wide_orientation = "v" if wide_x else "h"
             else:

packages/python/plotly/plotly/tests/test_core/test_px/test_px_wide.py

@@ -663,6 +663,19 @@ def append_special_case(df_in, args_in, args_expect, df_expect):
     ),
 )
 
+# y = columns
+df = pd.DataFrame(dict(a=[1, 2], b=[3, 4]), index=[7, 8])
+df.index.name = "c"
+df.columns.name = "d"
+append_special_case(
+    df_in=df,
+    args_in=dict(x=df.index, y=df.columns, color=None),
+    args_expect=dict(x="c", y="value", color="d"),
+    df_expect=pd.DataFrame(
+        dict(c=[7, 8, 7, 8], d=["a", "a", "b", "b"], value=[1, 2, 3, 4])
+    ),
+)
+
 
 @pytest.mark.parametrize("df_in, args_in, args_expect, df_expect", special_cases)
 def test_wide_mode_internal_special_cases(df_in, args_in, args_expect, df_expect):