Convert matplotlib stairs plots to plotly step lines

CHANGELOG.md

@@ -9,6 +9,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - 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 `mpl_to_plotly` dropping matplotlib stairs plots by rendering `StepPatch` artists as step lines or filled step areas from their path vertices and edge style [[#5698](https://github.com/plotly/plotly.py/pull/5698)], with thanks to @robertoffmoura for the contribution!
 - Fix `mpl_to_plotly` showing unwanted legends with placeholder or internal label entries (`_nolegend_`, `_childN`) by defaulting `layout.showlegend=False` unless the matplotlib figure explicitly defines a legend [[#5699](https://github.com/plotly/plotly.py/pull/5699)], with thanks to @robertoffmoura for the contribution!
 - Fix `mpl_to_plotly` crashing on figures with fully transparent or alpha-carrying colors, such as boxplot outlier markers (`facecolor="none"`), rgba line colors, and `color="none"` text and axis labels; all matplotlib colors are now exported through a single helper that maps `"none"` to transparent rgba and keeps or overrides alpha as matplotlib does [[#5700](https://github.com/plotly/plotly.py/pull/5700)], with thanks to @robertoffmoura for the contribution!
 - Export tick label colors to the plotly tickfont in `mpl_to_plotly` [[#5716](https://github.com/plotly/plotly.py/pull/5716)], with thanks to @robertoffmoura for the contribution!

plotly/matplotlylib/renderer.py

@@ -9,6 +9,7 @@
 
 import warnings
 
+import matplotlib.patches as mpatches
 import plotly.graph_objs as go
 from plotly.matplotlylib.mplexporter import Renderer
 from plotly.matplotlylib import mpltools
@@ -749,11 +750,12 @@ def _draw_filled_path_collection(self, props):
             )
 
     def draw_path(self, **props):
-        """Draw path, currently only attempts to draw bar charts.
+        """Draw a bar chart path or a matplotlib step patch.
 
         This function attempts to sort a given path into a collection of
-        horizontal or vertical bar charts. Most of the actual code takes
-        place in functions from mpltools.py.
+        horizontal or vertical bar charts, and draws matplotlib StepPatch
+        artists as step traces. Most of the actual code takes place in
+        functions from mpltools.py.
 
         props.keys() -- [
         'data',         (a list of vertices for the path)
@@ -777,13 +779,71 @@ def draw_path(self, **props):
         is_bar = mpltools.is_bar(self.current_mpl_ax.containers, **props)
         if is_bar:
             self.current_bars += [props]
+        elif isinstance(props["mplobj"], mpatches.StepPatch):
+            self.msg += "    Drawing a step path\n"
+            self._draw_step_path(props)
         else:
             self.msg += "    This path isn't a bar, not drawing\n"
             warnings.warn(
                 "I found a path object that I don't think is part "
                 "of a bar chart. Ignoring."
             )
 
+    def _draw_step_path(self, props):
+        """Draw a matplotlib StepPatch as a step trace."""
+        if props["coordinates"] != "data":
+            self.msg += "    Step path is not in data coordinates, not drawing\n"
+            return
+        style = props["style"]
+        segments = []
+        segment_x = []
+        segment_y = []
+        for (x0, y0), code in zip(props["data"], props["pathcodes"]):
+            if code == "M" and segment_x:
+                segments.append((segment_x, segment_y))
+                segment_x = []
+                segment_y = []
+            if not segment_x or x0 != segment_x[-1] or y0 != segment_y[-1]:
+                segment_x.append(x0)
+                segment_y.append(y0)
+        if segment_x:
+            segments.append((segment_x, segment_y))
+        x = []
+        y = []
+        for sub_x, sub_y in segments:
+            if x:
+                x.append(None)
+                y.append(None)
+            x.extend(self._convert_x_dates(sub_x))
+            y.extend(sub_y)
+        if len(x) < 2:
+            self.msg += "    Step path has fewer than 2 points, not drawing\n"
+            return
+        label = props["mplobj"].get_label()
+        if not label or (isinstance(label, str) and label.startswith("_")):
+            label = None
+            showlegend = False
+        else:
+            showlegend = None
+        self.plotly_fig.add_trace(
+            go.Scatter(
+                x=x,
+                y=y,
+                mode="lines",
+                name=label,
+                showlegend=showlegend,
+                line=go.scatter.Line(
+                    color=_export_color(style["edgecolor"], style["alpha"]),
+                    width=style["edgewidth"],
+                    dash=mpltools.convert_dash(style["dasharray"]),
+                ),
+                fill="toself" if style["facecolor"] != "none" else None,
+                fillcolor=_export_color(style["facecolor"]),
+                xaxis="x{0}".format(self.axis_ct),
+                yaxis="y{0}".format(self.axis_ct),
+            )
+        )
+
     def draw_text(self, **props):
         """Create an annotation dict for a text obj.
 

plotly/matplotlylib/tests/test_renderer.py

@@ -542,6 +542,101 @@ def test_background_colors_from_matplotlib_defaults():
     assert plotly_fig.layout.paper_bgcolor == "#FFFFFF"
 
 
+def test_stairs_converts_to_step_line():
+    fig, ax = plt.subplots()
+    ax.stairs([0.0, 1.0, 0.0], [0.0, 1.0, 2.0, 3.0])
+    plotly_fig = tls.mpl_to_plotly(fig)
+    assert len(plotly_fig.data) == 1
+    trace = plotly_fig.data[0]
+    assert trace.mode == "lines"
+    assert tuple(trace.x) == (0.0, 1.0, 1.0, 2.0, 2.0, 3.0)
+    assert tuple(trace.y) == (0.0, 0.0, 1.0, 1.0, 0.0, 0.0)
+
+
+def test_stairs_date_xaxis():
+    """Stairs with date x-values must export date strings."""
+    dates = [
+        datetime.datetime(2023, 1, 1) + datetime.timedelta(days=i) for i in range(4)
+    ]
+    fig, ax = plt.subplots()
+    ax.stairs([0.0, 1.0, 0.0], dates)
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.xaxis.type == "date"
+    trace = plotly_fig.data[0]
+    assert all(isinstance(x, str) for x in trace.x)
+
+
+def test_stairs_fill_converts_to_filled_area():
+    """Filled stairs must export the patch facecolor as a filled area."""
+    fig, ax = plt.subplots()
+    ax.stairs([0.0, 1.0, 0.0], [0.0, 1.0, 2.0, 3.0], fill=True)
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    trace = plotly_fig.data[0]
+    assert trace.fill == "toself"
+    assert trace.fillcolor == "#1F77B4"
+    assert tuple(trace.x) == (0.0, 1.0, 1.0, 2.0, 2.0, 3.0)
+    assert tuple(trace.y) == (0.0, 0.0, 1.0, 1.0, 0.0, 0.0)
+
+
+def test_stairs_nan_values_split_into_disjoint_steps():
+    """NaN values split stairs into separate step regions."""
+    fig, ax = plt.subplots()
+    ax.stairs([1.0, np.nan, 0.5], [0.0, 1.0, 2.0, 3.0], baseline=0.2)
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    trace = plotly_fig.data[0]
+    assert tuple(trace.x) == (0.0, 0.0, 1.0, 1.0, None, 2.0, 2.0, 3.0, 3.0)
+    assert tuple(trace.y) == (0.2, 1.0, 1.0, 0.2, None, 0.2, 0.5, 0.5, 0.2)
+
+
+def test_stairs_date_xaxis_with_nan_values():
+    """NaN-separated stairs on a date axis must export date strings."""
+    dates = [
+        datetime.datetime(2023, 1, 1) + datetime.timedelta(days=i) for i in range(4)
+    ]
+    fig, ax = plt.subplots()
+    ax.stairs([1.0, np.nan, 0.5], dates)
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    trace = plotly_fig.data[0]
+    assert None in trace.x
+    assert all(isinstance(x, str) or x is None for x in trace.x)
+
+
+def test_stairs_label_used_in_legend():
+    """A labeled stairs patch must become a named legend entry."""
+    fig, ax = plt.subplots()
+    ax.stairs([0.0, 1.0, 0.0], [0.0, 1.0, 2.0, 3.0], label="my stairs")
+    ax.legend()
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.showlegend is True
+    assert plotly_fig.data[0].name == "my stairs"
+    assert plotly_fig.data[0].showlegend is not False
+
+
+def test_stairs_without_label_hidden_from_legend():
+    """An unlabeled stairs patch must not appear in the plotly legend."""
+    fig, ax = plt.subplots()
+    ax.stairs([0.0, 1.0, 0.0], [0.0, 1.0, 2.0, 3.0])
+    ax.plot([0, 1], [2, 2], label="Labeled line")
+    ax.legend()
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.showlegend is True
+    stairs = [trace for trace in plotly_fig.data if trace.name is None]
+    assert len(stairs) == 1
+    assert stairs[0].showlegend is False
+
+
 def test_custom_background_colors_are_preserved():
     fig, ax = plt.subplots()
     fig.patch.set_facecolor("lightyellow")