Add legend.linewidth parameter to control legend box edge linewidth (matplotlib#30780)

* Added legend.linewidth parameter to control legend box edge linewidth

* Added legend.linewidth parameter to control legend box edge linewidth

* linting issue and mypy subtest issue fixed

* suggestions implemented

* tests added

* suggestion taken and .rst file renamed

* Fix linting error

---------

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

Author: manit2004

doc/release/next_whats_new/legend_line_width.rst

@@ -0,0 +1,21 @@
+``legend.linewidth`` rcParam and parameter
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A new rcParam ``legend.linewidth`` has been added to control the line width of
+the legend's box edges. When set to ``None`` (the default), it inherits the
+value from ``patch.linewidth``. This allows for independent control of the
+legend frame line width without affecting other elements.
+
+The `.Legend` constructor also accepts a new *linewidth* parameter to set the
+legend frame line width directly, overriding the rcParam value.
+
+.. plot::
+    :include-source: true
+    :alt: A line plot with a legend showing a thick border around the legend box.
+
+    import matplotlib.pyplot as plt
+
+    fig, ax = plt.subplots()
+    ax.plot([1, 2, 3], label='data')
+    ax.legend(linewidth=2.0)  # Thick legend box edge
+    plt.show()

lib/matplotlib/legend.py

@@ -196,6 +196,12 @@ def _update_bbox_to_anchor(self, loc_in_canvas):
     The legend's background patch edge color.
     If ``"inherit"``, use :rc:`axes.edgecolor`.
 
+linewidth : float or None, default: :rc:`legend.linewidth`
+    The legend's background patch edge linewidth.
+    If ``None``, use :rc:`patch.linewidth`.
+
+    .. versionadded:: 3.11
+
 mode : {"expand", None}
     If *mode* is set to ``"expand"`` the legend will be horizontally
     expanded to fill the Axes area (or *bbox_to_anchor* if defines
@@ -385,6 +391,7 @@ def __init__(
         framealpha=None,      # set frame alpha
         edgecolor=None,       # frame patch edgecolor
         facecolor=None,       # frame patch facecolor
+        linewidth=None,       # frame patch linewidth
 
         bbox_to_anchor=None,  # bbox to which the legend will be anchored
         bbox_transform=None,  # transform for the bbox
@@ -526,9 +533,12 @@ def __init__(
 
         fancybox = mpl._val_or_rc(fancybox, "legend.fancybox")
 
+        linewidth = mpl._val_or_rc(linewidth, "legend.linewidth")
+
         self.legendPatch = FancyBboxPatch(
             xy=(0, 0), width=1, height=1,
             facecolor=facecolor, edgecolor=edgecolor,
+            linewidth=linewidth,
             # If shadow is used, default to alpha=1 (#8943).
             alpha=(framealpha if framealpha is not None
                    else 1 if shadow

lib/matplotlib/legend.pyi

@@ -85,6 +85,7 @@ class Legend(Artist):
         framealpha: float | None = ...,
         edgecolor: Literal["inherit"] | ColorType | None = ...,
         facecolor: Literal["inherit"] | ColorType | None = ...,
+        linewidth: float | None = ...,
         bbox_to_anchor: BboxBase
         | tuple[float, float]
         | tuple[float, float, float, float]

lib/matplotlib/mpl-data/matplotlibrc

@@ -562,6 +562,7 @@
 #legend.framealpha:    0.8      # legend patch transparency
 #legend.facecolor:     inherit  # inherit from axes.facecolor; or color spec
 #legend.edgecolor:     0.8      # background patch boundary color
+#legend.linewidth:     None     # line width of the legend frame, None means inherit from patch.linewidth
 #legend.fancybox:      True     # if True, use a rounded box for the
                                 # legend background, else a rectangle
 #legend.shadow:        False    # if True, give background a shadow effect

lib/matplotlib/rcsetup.py

@@ -1184,6 +1184,8 @@ def _convert_validator_spec(key, conv):
     "legend.frameon":        validate_bool,
     # alpha value of the legend frame
     "legend.framealpha":     validate_float_or_None,
+    # linewidth of legend frame
+    "legend.linewidth": validate_float_or_None,
 
     ## the following dimensions are in fraction of the font size
     "legend.borderpad":      validate_float,  # units are fontsize

lib/matplotlib/tests/test_legend.py

@@ -1669,6 +1669,37 @@ def test_boxplot_legend_labels():
     assert all(x.get_label().startswith("_") for x in bp4['medians'][1:])
 
 
+def test_legend_linewidth():
+    """Test legend.linewidth parameter and rcParam."""
+    fig, ax = plt.subplots()
+    ax.plot([1, 2, 3], label='data')
+
+    # Test direct parameter
+    leg = ax.legend(linewidth=2.5)
+    assert leg.legendPatch.get_linewidth() == 2.5
+
+    # Test rcParam
+    with mpl.rc_context({'legend.linewidth': 3.0}):
+        fig, ax = plt.subplots()
+        ax.plot([1, 2, 3], label='data')
+        leg = ax.legend()
+        assert leg.legendPatch.get_linewidth() == 3.0
+
+    # Test None default (should inherit from patch.linewidth)
+    with mpl.rc_context({'legend.linewidth': None, 'patch.linewidth': 1.5}):
+        fig, ax = plt.subplots()
+        ax.plot([1, 2, 3], label='data')
+        leg = ax.legend()
+        assert leg.legendPatch.get_linewidth() == 1.5
+
+    # Test that direct parameter overrides rcParam
+    with mpl.rc_context({'legend.linewidth': 1.0}):
+        fig, ax = plt.subplots()
+        ax.plot([1, 2, 3], label='data')
+        leg = ax.legend(linewidth=4.0)
+        assert leg.legendPatch.get_linewidth() == 4.0
+
+
 def test_patchcollection_legend():
     # Test that PatchCollection labels show up in legend and preserve visual
     # properties (issue #23998)