Density Matrix plotting is being fixed (quantumlib#4805)

The plotting positions were dividing by cell size instead of multiplying, fixed that.
Closes quantumlib#4804.

cirq-core/cirq/vis/density_matrix.py

@@ -41,18 +41,18 @@ def _plot_element_of_density_matrix(ax, x, y, r, phase, show_rect=False, show_te
     _image_opacity = 0.8 if not show_text else 0.4
 
     circle_out = plt.Circle(
-        (x + 0.5, y + 0.5), radius=1 / _half_cell_size_after_padding, fill=False, color='#333333'
+        (x + 0.5, y + 0.5), radius=1 * _half_cell_size_after_padding, fill=False, color='#333333'
     )
     circle_in = plt.Circle(
         (x + 0.5, y + 0.5),
-        radius=r / _half_cell_size_after_padding,
+        radius=r * _half_cell_size_after_padding,
         fill=True,
         color='IndianRed',
         alpha=_image_opacity,
     )
     line = lines.Line2D(
-        (x + 0.5, x + 0.5 + np.cos(phase) / _half_cell_size_after_padding),
-        (y + 0.5, y + 0.5 + np.sin(phase) / _half_cell_size_after_padding),
+        (x + 0.5, x + 0.5 + np.cos(phase) * _half_cell_size_after_padding),
+        (y + 0.5, y + 0.5 + np.sin(phase) * _half_cell_size_after_padding),
         color='#333333',
         alpha=_image_opacity,
     )
@@ -128,7 +128,7 @@ def plot_density_matrix(
         f"{'0'*(num_qubits - len(f'{i:b}'))}{i:b}" for i in range(matrix.shape[0])
     ]
     ax.set_xticks(ticks)
-    ax.set_xticklabels(labels)
+    ax.set_xticklabels(labels, rotation=90)
     ax.set_yticks(ticks)
     ax.set_yticklabels(reversed(labels))
     ax.set_facecolor('#eeeeee')

cirq-core/cirq/vis/density_matrix_test.py

@@ -49,9 +49,8 @@ def test_density_matrix_plotter(size, show_text):
 
 @pytest.mark.parametrize('show_text', [True, False])
 @pytest.mark.parametrize('size', [2, 4, 8, 16])
-def test_density_matrix_circle_sizes(size, show_text):
+def test_density_matrix_circle_rectangle_sizes(size, show_text):
     matrix = cirq.testing.random_density_matrix(size)
-    # Check that the correct title is being shown
     ax = plot_density_matrix(matrix, show_text=show_text, title='Test Density Matrix Plot')
     # Check that the radius of all the circles in the matrix is correct
     circles = list(filter(lambda x: isinstance(x, patches.Circle), ax.get_children()))
@@ -90,6 +89,29 @@ def test_density_matrix_circle_sizes(size, show_text):
         )
 
 
+@pytest.mark.parametrize('show_text', [True, False])
+@pytest.mark.parametrize('size', [2, 4, 8, 16])
+def test_density_matrix_sizes_upper_bounds(size, show_text):
+    matrix = cirq.testing.random_density_matrix(size)
+    ax = plot_density_matrix(matrix, show_text=show_text, title='Test Density Matrix Plot')
+
+    circles = list(filter(lambda x: isinstance(x, patches.Circle), ax.get_children()))
+    max_radius = np.max([c.radius for c in circles if c.fill])
+
+    rects = list(
+        filter(
+            lambda x: isinstance(x, patches.Rectangle) and x.get_alpha() is not None,
+            ax.get_children(),
+        )
+    )
+    max_height = np.max([r.get_height() for r in rects])
+    max_width = np.max([r.get_width() for r in rects])
+
+    assert max_height <= 1.0, "Some rectangle is exceeding out of it's cell size"
+    assert max_width <= 1.0, "Some rectangle is exceeding out of it's cell size"
+    assert max_radius * 2 <= 1.0, "Some circle is exceeding out of it's cell size"
+
+
 @pytest.mark.parametrize('show_rect', [True, False])
 @pytest.mark.parametrize('value', [0.0, 1.0, 0.5 + 0.3j, 0.2 + 0.1j, 0.5 + 0.5j])
 def test_density_element_plot(value, show_rect):