FIX: Make stem() baseline follow the curvature in polar plots

Note: There's precedence for a similar special-casing on polar Axes for errorbars:
https://github.com/matplotlib/matplotlib/blob/3fb9c0961b5c8d5753c88ac37c08cda58a4b7839/lib/matplotlib/axes/_axes.py#L3798

lib/matplotlib/axes/_axes.py

@@ -3186,6 +3186,10 @@ def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0,
             marker_y = heads
             baseline_x = [np.min(locs), np.max(locs)]
             baseline_y = [bottom, bottom]
+        if self.name == "polar":
+            # The baseline must be interpolated to follow the curvature
+            baseline_x = np.linspace(*baseline_x, 200)
+            baseline_y = np.linspace(*baseline_y, 200)
 
         markerline, = self.plot(marker_x, marker_y,
                                 color=markercolor, linestyle=markerstyle,

lib/matplotlib/tests/test_axes.py

@@ -4617,6 +4617,23 @@ def test_stem_orientation():
             orientation='horizontal')
 
 
+def test_stem_polar_baseline():
+    """Test that the baseline is sampled so that it will follow the radius."""
+    fig = plt.figure()
+    ax = fig.add_subplot(projection='polar')
+    x = np.linspace(1.57, 3.14, 10)
+    y = np.linspace(0, 1, 10)
+    bottom = 0.5
+    container = ax.stem(x, y, bottom=bottom)
+    baseline_x, baseline_y = container.baseline.get_data()
+    # The number of baseline points N_BASELINE_POINTS is an implementation detail.
+    # Nevertheless, it's simpler and clearer to test with this detail rather than
+    # to generically test "sufficiently high sampled".
+    N_BASELINE_POINTS = 200
+    assert_allclose(baseline_x, np.linspace(x.min(), x.max(), N_BASELINE_POINTS))
+    assert_allclose(baseline_y, np.full(N_BASELINE_POINTS, bottom)
+
+
 @image_comparison(['hist_stacked_stepfilled_alpha'])
 def test_hist_stacked_stepfilled_alpha():
     # make some data