feat: correct ax.legend() order for stack hists

src/mplhep/plot.py

@@ -409,6 +409,20 @@ def iterable_not_string(arg):
     ##########
     # Plotting
     return_artists: list[StairsArtists | ErrorBarArtists] = []
+    # customize color cycle assignment when stacking to match legend
+    if stack:
+        plottables = plottables[::-1]
+        _chunked_kwargs = _chunked_kwargs[::-1]
+        _labels = _labels[::-1]
+        if "color" not in kwargs:
+            # Inverse default color cycle
+            _colors = []
+            for _ in range(len(plottables)):
+                _colors.append(ax._get_lines.get_next_color())  # type: ignore[attr-defined]
+            _colors.reverse()
+            for i in range(len(plottables)):
+                _chunked_kwargs[i].update({"color": _colors[i]})
+
     if histtype == "step":
         for i in range(len(plottables)):
             do_errors = yerr is not False and (
@@ -419,6 +433,7 @@ def iterable_not_string(arg):
             _kwargs = _chunked_kwargs[i]
             _label = _labels[i] if do_errors else None
             _step_label = _labels[i] if not do_errors else None
+
             _kwargs = soft_update_kwargs(_kwargs, {"linewidth": 1.5})
 
             _plot_info = plottables[i].to_stairs()

tests/test_mock.py

@@ -28,6 +28,11 @@ def mock_matplotlib(mocker):
     ax.plot.return_value = (line2d,)
     ax.errorbar.return_value = (line2d,)
 
+    # Mock the _get_lines attribute
+    _get_lines = mocker.Mock()
+    _get_lines.get_next_color.return_value = "next-color"
+    ax._get_lines = _get_lines
+
     mpl = mocker.patch("matplotlib.pyplot", autospec=True)
     mocker.patch("matplotlib.pyplot.subplots", return_value=(fig, ax))
 
@@ -93,7 +98,7 @@ def test_histplot_real(mock_matplotlib):
     hep.histplot([c], bins=bins, ax=ax, yerr=True, histtype="errorbar", label="Data")
     ax.legend()
     ax.set_title("Data/MC")
-    assert len(ax.mock_calls) == 18
+    assert len(ax.mock_calls) == 20
     ax.reset_mock()
 
     hep.histplot(
@@ -110,7 +115,7 @@ def test_histplot_real(mock_matplotlib):
     )
     ax.legend()
     ax.set_title("Data/MC binwnorm")
-    assert len(ax.mock_calls) == 18
+    assert len(ax.mock_calls) == 20
     ax.reset_mock()
 
     hep.histplot(
@@ -127,4 +132,4 @@ def test_histplot_real(mock_matplotlib):
     )
     ax.legend()
     ax.set_title("Data/MC Density")
-    assert len(ax.mock_calls) == 18
+    assert len(ax.mock_calls) == 20