Preserve matplotlib axis line colors when converting from matplotlib

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 - Fix `mpl_to_plotly` tick marker mirroring, axis positioning, and trace visibility for twinned and overlaid subplots (such as `twinx` and `twiny`) [[#5310](https://github.com/plotly/plotly.py/pull/5310)], with thanks to @robertoffmoura for the contribution!
+- Fix `mpl_to_plotly` losing matplotlib axis line colors by exporting the color of each axis's displayed spine to the plotly x and y axis `linecolor` [[#5311](https://github.com/plotly/plotly.py/pull/5311)], with thanks to @robertoffmoura for the contribution!
 - 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!

plotly/matplotlylib/renderer.py

@@ -210,6 +210,10 @@ def open_axes(self, ax, props):
         )
         xaxis.update(mpl_xaxis)
         yaxis.update(mpl_yaxis)
+        x_spine = "top" if xaxis["side"] == "top" else "bottom"
+        y_spine = "right" if yaxis["side"] == "right" else "left"
+        xaxis["linecolor"] = _export_color(ax.spines[x_spine].get_edgecolor())
+        yaxis["linecolor"] = _export_color(ax.spines[y_spine].get_edgecolor())
         bottom_spine = mpltools.get_spine_visible(ax, "bottom")
         top_spine = mpltools.get_spine_visible(ax, "top")
         left_spine = mpltools.get_spine_visible(ax, "left")

plotly/matplotlylib/tests/test_renderer.py

@@ -518,6 +518,54 @@ def test_contourf_bands_render():
     assert len(filled) > 0
 
 
+def test_axis_linecolor_defaults_to_black():
+    fig, ax = plt.subplots()
+    ax.plot([0, 1], [0, 1])
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.xaxis.linecolor == "#000000"
+    assert plotly_fig.layout.yaxis.linecolor == "#000000"
+
+
+def test_custom_axis_linecolors_are_preserved():
+    fig, ax = plt.subplots()
+    ax.spines["bottom"].set_color("red")
+    ax.spines["left"].set_color("green")
+    ax.plot([0, 1], [0, 1])
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.xaxis.linecolor == "#FF0000"
+    assert plotly_fig.layout.yaxis.linecolor == "#008000"
+
+
+def test_axis_linecolor_follows_right_axis_side():
+    fig, ax = plt.subplots()
+    ax.plot([0, 1], [0, 1])
+    ax2 = ax.twinx()
+    ax2.plot([0, 1], [1, 0])
+    ax2.spines["right"].set_color("red")
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.yaxis2.side == "right"
+    assert plotly_fig.layout.yaxis2.linecolor == "#FF0000"
+
+
+def test_axis_linecolor_follows_top_axis_side():
+    fig, ax = plt.subplots()
+    ax.spines["bottom"].set_visible(False)
+    ax.spines["top"].set_color("red")
+    ax.xaxis.set_ticks_position("top")
+    ax.plot([0, 1], [0, 1])
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.xaxis.side == "top"
+    assert plotly_fig.layout.xaxis.linecolor == "#FF0000"
+
+
 def test_filled_path_collection_date_xaxis():
     """Filled path collections with date x-values must export date strings,
     not raw matplotlib date numbers."""