Convert matplotlib step drawstyles to plotly line shapes

CHANGELOG.md

@@ -7,6 +7,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 - Fix concurrent first access to lazily initialized graph object properties, which could raise `ValueError("Invalid value")` [[#5691](https://github.com/plotly/plotly.py/pull/5691)], with thanks to @hb1915 for the contribution!
 - Fix `mpl_to_plotly` crashing on touching bars (such as `plt.hist`) due to floating-point noise producing negative `bargap` values by clamping `bargap` to `[0, 1]` [[#5696](https://github.com/plotly/plotly.py/pull/5696)], with thanks to @robertoffmoura for the contribution!
+- Fix `mpl_to_plotly` drawing step plots with diagonal lines instead of vertical/horizontal step segments by mapping matplotlib step drawstyles (`steps-pre`, `steps-post`, `steps-mid`) to plotly line shapes [[#5697](https://github.com/plotly/plotly.py/pull/5697)], with thanks to @robertoffmoura for the contribution!
 - Fix `px.sunburst`, `px.treemap` and `px.icicle` listing sectors in a different order on every run when `path` is used with a Polars DataFrame; sectors now follow their order of first appearance for all dataframe backends [[#5766](https://github.com/plotly/plotly.py/pull/5766)], with thanks to @Irahan2 for the contribution!
 
 ## [7.1.0] - 2026-09-15

plotly/matplotlylib/mpltools.py

@@ -278,6 +278,23 @@ def get_bar_gap(bar_starts, bar_ends, tol=1e-10):
             return min(max(gap0 / bar_delta, 0.0), 1.0)
 
 
+DRAWSTYLE_SHAPE_MAP = {
+    "steps": "vh",
+    "steps-pre": "vh",
+    "steps-post": "hv",
+    "steps-mid": "hvh",
+}
+
+
+def convert_drawstyle(drawstyle):
+    """Convert a matplotlib line drawstyle to a plotly line shape.
+
+    Matplotlib draws steps as vertical/horizontal segments; plotly's
+    ``line.shape`` expresses the same via "vh", "hv" and "hvh".
+    """
+    return DRAWSTYLE_SHAPE_MAP.get(drawstyle)
+
+
 def convert_rgba_array(color_list):
     clean_color_list = list()
     for c in color_list:

plotly/matplotlylib/renderer.py

@@ -407,6 +407,7 @@ def draw_marked_line(self, **props):
                     color=color,
                     width=props["linestyle"]["linewidth"],
                     dash=mpltools.convert_dash(props["linestyle"]["dasharray"]),
+                    shape=mpltools.convert_drawstyle(props["linestyle"]["drawstyle"]),
                 )
             else:
                 shape = dict(

plotly/matplotlylib/tests/test_renderer.py

@@ -37,6 +37,22 @@ def test_no_fake_legend_shapes_with_native_legend():
     assert len(plotly_fig.layout.annotations) == 0
 
 
+def test_drawstyle_maps_to_line_shape():
+    cases = {
+        "steps-pre": "vh",
+        "steps": "vh",
+        "steps-post": "hv",
+        "steps-mid": "hvh",
+    }
+    for drawstyle, shape in cases.items():
+        fig, ax = plt.subplots()
+        ax.plot([0, 1, 2], [0, 1, 0], drawstyle=drawstyle)
+
+        plotly_fig = tls.mpl_to_plotly(fig)
+
+        assert plotly_fig.data[0].line.shape == shape
+
+
 def test_legend_disabled_when_no_matplotlib_legend():
     """Test that legend is not enabled when no matplotlib legend is present."""
     fig, ax = plt.subplots()