Fix Axes.grid() to respect alpha in color tuples

Nabeelshar · Nabeelshar · commit f6513e7c04a2 · 2025-12-13T11:01:34.000+05:00
When calling ax.grid(color=(R, G, B, A)) with a 4-element color tuple, the alpha channel was being ignored, causing gridlines to appear fully opaque instead of with the specified transparency. The issue occurred in two locations: 1. Tick.__init__: When creating ticks, if grid_alpha was None but the color had an alpha channel, the alpha wasn't extracted from the color 2. Tick._apply_params: When updating gridlines via grid(), colors with alpha weren't being decomposed into separate color and alpha params This fix extracts the alpha value from color tuples using mcolors.to_rgba() and sets both the color (RGB only) and alpha separately, ensuring proper transparency rendering. Fixes matplotlib#22231
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
@@ -140,17 +140,20 @@ def __init__(
             f"grid.{major_minor}.linewidth",
             "grid.linewidth",
         )
-        if grid_alpha is None and not mcolors._has_alpha_channel(grid_color):
-            # alpha precedence: kwarg > color alpha > rcParams['grid.alpha']
-            # Note: only resolve to rcParams if the color does not have alpha
-            # otherwise `grid(color=(1, 1, 1, 0.5))` would work like
-            #   grid(color=(1, 1, 1, 0.5), alpha=rcParams['grid.alpha'])
-            # so the that the rcParams default would override color alpha.
-            grid_alpha = mpl._val_or_rc(
-                # grid_alpha is None so we can use the first key
-                mpl.rcParams[f"grid.{major_minor}.alpha"],
-                "grid.alpha",
-            )
+        if grid_alpha is None:
+            if mcolors._has_alpha_channel(grid_color):
+                # Extract alpha from the color
+                # alpha precedence: kwarg > color alpha > rcParams['grid.alpha']
+                rgba = mcolors.to_rgba(grid_color)
+                grid_color = rgba[:3]  # RGB only
+                grid_alpha = rgba[3]    # Alpha from color
+            else:
+                # No alpha in color, use rcParams
+                grid_alpha = mpl._val_or_rc(
+                    # grid_alpha is None so we can use the first key
+                    mpl.rcParams[f"grid.{major_minor}.alpha"],
+                    "grid.alpha",
+                )
 
         grid_kw = {k[5:]: v for k, v in kwargs.items() if k != "rotation_mode"}
 
@@ -348,6 +351,15 @@ def _apply_params(self, **kwargs):
 
         grid_kw = {k[5:]: v for k, v in kwargs.items()
                    if k in _gridline_param_names}
+        # If grid_color has an alpha channel and grid_alpha is not explicitly
+        # set, extract the alpha from the color.
+        if 'color' in grid_kw and 'alpha' not in grid_kw:
+            grid_color = grid_kw['color']
+            if mcolors._has_alpha_channel(grid_color):
+                # Convert to rgba to extract alpha
+                rgba = mcolors.to_rgba(grid_color)
+                grid_kw['color'] = rgba[:3]  # RGB only
+                grid_kw['alpha'] = rgba[3]    # Alpha channel
         self.gridline.set(**grid_kw)
 
     def update_position(self, loc):
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -6133,6 +6133,28 @@ def test_grid():
     assert not ax.xaxis.majorTicks[0].gridline.get_visible()
 
 
+def test_grid_color_with_alpha():
+    """Test that grid(color=(..., alpha)) respects the alpha value."""
+    fig, ax = plt.subplots()
+    ax.plot([0, 1], [0, 1])
+    
+    # Test 1: Color tuple with alpha
+    ax.grid(True, color=(0.5, 0.6, 0.7, 0.3))
+    fig.canvas.draw()
+    
+    # Check that alpha is extracted from color tuple
+    for tick in ax.xaxis.get_major_ticks():
+        assert tick.gridline.get_alpha() == 0.3, \
+            f"Expected alpha=0.3, got {tick.gridline.get_alpha()}"
+        # Color should be RGB only, without alpha component
+        color = tick.gridline.get_color()
+        assert len(color) in (3, 4), f"Unexpected color format: {color}"
+        
+    for tick in ax.yaxis.get_major_ticks():
+        assert tick.gridline.get_alpha() == 0.3, \
+            f"Expected alpha=0.3, got {tick.gridline.get_alpha()}"
+
+
 def test_reset_grid():
     fig, ax = plt.subplots()
     ax.tick_params(reset=True, which='major', labelsize=10)
