Fix center of rotation with rotation_mode='anchor'

Wiliam · QuLogic · commit c44db77b9fb1 · 2025-06-10T16:50:09.000-04:00
diff --git a/lib/matplotlib/backends/backend_agg.py b/lib/matplotlib/backends/backend_agg.py
@@ -197,10 +197,20 @@ def draw_text(self, gc, x, y, s, prop, angle, ismath=False, mtext=None):
         xo, yo = font.get_bitmap_offset()
         xo /= 64.0
         yo /= 64.0
-        xd = d * sin(radians(angle))
-        yd = d * cos(radians(angle))
-        x = round(x + xo + xd)
-        y = round(y + yo + yd)
+
+        rad = radians(angle)
+        xd = d * sin(rad)
+        yd = d * cos(rad)
+        # Rotating the offset vector ensures text rotates around the anchor point.
+        # Without this, rotated text offsets incorrectly, causing a horizontal shift.
+        # Applying the 2D rotation matrix.
+        rotated_xo = xo * cos(rad) - yo * sin(rad)
+        rotated_yo = xo * sin(rad) + yo * cos(rad)
+        # Subtract rotated_yo to account for the inverted y-axis in computer graphics,
+        # compared to the mathematical convention.
+        x = round(x + rotated_xo + xd)
+        y = round(y - rotated_yo + yd)
+
         self._renderer.draw_text_image(font, x, y + 1, angle, gc)
 
     def get_text_width_height_descent(self, s, prop, ismath):
diff --git a/lib/matplotlib/tests/baseline_images/test_text/rotation_anchor.png b/lib/matplotlib/tests/baseline_images/test_text/rotation_anchor.png
diff --git a/lib/matplotlib/tests/test_text.py b/lib/matplotlib/tests/test_text.py
@@ -301,6 +301,22 @@ def test_alignment():
     ax.set_yticks([])
 
 
+@image_comparison(baseline_images=['rotation_anchor.png'], style='mpl20',
+                  remove_text=True)
+def test_rotation_mode_anchor():
+    fig, ax = plt.subplots()
+
+    ax.plot([0, 1], lw=0)
+    ax.axvline(.5, linewidth=.5, color='.5')
+    ax.axhline(.5, linewidth=.5, color='.5')
+
+    N = 4
+    for r in range(N):
+        ax.text(.5, .5, 'pP', color=f'C{r}', size=100,
+                rotation=r/N*360, rotation_mode='anchor',
+                verticalalignment='center_baseline')
+
+
 @image_comparison(['axes_titles.png'])
 def test_axes_titles():
     # Related to issue #3327
