set line_group in wide mode

Author: nicolaskruchten

CHANGELOG.md

@@ -9,6 +9,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fixed special cases with `px.sunburst` and `px.treemap` with `path` input ([#2524](https://github.com/plotly/plotly.py/pull/2524))
 - Fixed bug in `hover_data` argument of `px` functions, when the column name is changed with labels and `hover_data` is a dictionary setting up a specific format for the hover data ([#2544](https://github.com/plotly/plotly.py/pull/2544)).
 - Made the Plotly Express `trendline` argument more robust and made it work with datetime `x` values ([#2554](https://github.com/plotly/plotly.py/pull/2554))
+- `px.line` now sets `line_group=<variable>` in wide mode by default ([#2599](https://github.com/plotly/plotly.py/pull/2599))
 
 ## [4.8.1] - 2020-05-28
 

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

@@ -1416,6 +1416,8 @@ def build_dataframe(args, constructor):
             args["y" if orient_v else "x"] = value_name
             if constructor != go.Histogram2d:
                 args["color"] = args["color"] or var_name
+            if "line_group" in args:
+                args["line_group"] = args["line_group"] or var_name
         if constructor == go.Bar:
             if _is_continuous(df_output, value_name):
                 args["x" if orient_v else "y"] = wide_cross_name

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

@@ -742,3 +742,20 @@ def test_mixed_input_error(df):
         "Plotly Express cannot process wide-form data with columns of different type"
         in str(err_msg.value)
     )
+
+
+def test_line_group():
+    df = pd.DataFrame(
+        data={
+            "who": ["a", "a", "b", "b"],
+            "x": [0, 1, 0, 1],
+            "score": [1.0, 2, 3, 4],
+            "miss": [3.2, 2.5, 1.3, 1.5],
+        }
+    )
+    fig = px.line(df, x="x", y=["miss", "score"])
+    assert len(fig.data) == 2
+    fig = px.line(df, x="x", y=["miss", "score"], color="who")
+    assert len(fig.data) == 4
+    fig = px.scatter(df, x="x", y=["miss", "score"], color="who")
+    assert len(fig.data) == 2