Set device pixel ratio when saving a figure

QuLogic · QuLogic · commit 630f25f6b1c4 · 2021-09-13T16:51:43.000-04:00
Otherwise, `FigureCanvasBase.get_width_height` returns a size scaled
down by the current pixel ratio, even though the DPI is not scaled up.
This causes the saved figure to be cropped.
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -2259,6 +2259,7 @@ def print_figure(
         # Remove the figure manager, if any, to avoid resizing the GUI widget.
         with cbook._setattr_cm(self, manager=None), \
              cbook._setattr_cm(self.figure, dpi=dpi), \
+             cbook._setattr_cm(canvas, _device_pixel_ratio=1), \
              cbook._setattr_cm(canvas, _is_saving=True), \
              ExitStack() as stack:
 
diff --git a/lib/matplotlib/tests/test_figure.py b/lib/matplotlib/tests/test_figure.py
@@ -6,6 +6,10 @@
 from types import SimpleNamespace
 import warnings
 
+import numpy as np
+import pytest
+from PIL import Image
+
 import matplotlib as mpl
 from matplotlib import cbook, rcParams
 from matplotlib._api.deprecation import MatplotlibDeprecationWarning
@@ -16,8 +20,6 @@
 import matplotlib.pyplot as plt
 import matplotlib.dates as mdates
 import matplotlib.gridspec as gridspec
-import numpy as np
-import pytest
 
 
 @image_comparison(['figure_align_labels'], extensions=['png', 'svg'],
@@ -496,6 +498,29 @@ def test_savefig_backend():
         fig.savefig("test.png", backend="pdf")
 
 
+@pytest.mark.parametrize('backend', [
+    pytest.param('Agg', marks=[pytest.mark.backend('Agg')]),
+    pytest.param('Cairo', marks=[pytest.mark.backend('Cairo')]),
+])
+def test_savefig_pixel_ratio(backend):
+    fig, ax = plt.subplots()
+    ax.plot([1, 2, 3])
+    with io.BytesIO() as buf:
+        fig.savefig(buf, format='png')
+        ratio1 = Image.open(buf)
+        ratio1.load()
+
+    fig, ax = plt.subplots()
+    ax.plot([1, 2, 3])
+    fig.canvas._set_device_pixel_ratio(2)
+    with io.BytesIO() as buf:
+        fig.savefig(buf, format='png')
+        ratio2 = Image.open(buf)
+        ratio2.load()
+
+    assert ratio1 == ratio2
+
+
 def test_figure_repr():
     fig = plt.figure(figsize=(10, 20), dpi=10)
     assert repr(fig) == "<Figure size 100x200 with 0 Axes>"
