Support individual styling of major and minor grid through rcParams (matplotlib#29481)

* Possible fix for issue matplotlib#13919

* fix flake warnings

* Resolve suggested changes

* Add new validators in stub file

* make stub file arguments same as runtime

* Add distinction for x and y axis in grid line options

* fix flake8

* Add What's New note

* Extend `_val_or_rc` to support multiply names
`_val_or_rc` now accept multiple rc names and return val or the first non-None value in rcParams. Returns last rc name if all other are None. Also, simplified code in `Tick` for grid lines creatation

* Fix linting for and _validate_linestyle_or_None to mypy allowlist

* Remove validate linestyle functions from stubtest allow list

* Revert change to just grid.major/minor distinction in rcParams

* Update What's New note and reduced example

* Add testing for `grid.major/minor.*` in rcParams

* fix indentation and linting

* Fix example description

* Fix spelling

* Fix type

* Fix formatting
Removed outer brackets

* `validate_color_or_None` private and fix argument names

* Fix validator name in stub file as well

* correct validators for grid.*.color keys

* Revert change in mypy-stubtest-allowlist.txt

---------

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

Author: konmenel

doc/users/next_whats_new/new_rcparams_grid_options.rst

@@ -0,0 +1,33 @@
+Separate styling options for major/minor grid line in rcParams
+--------------------------------------------------------------
+
+Using :rc:`grid.major.*` or :rc:`grid.minor.*` will overwrite the value in
+:rc:`grid.*` for the major and minor gridlines, respectively.
+
+.. plot::
+    :include-source: true
+    :alt: Modifying the gridlines using the new options `rcParams`
+
+    import matplotlib as mpl
+    import matplotlib.pyplot as plt
+
+
+    # Set visibility for major and minor gridlines
+    mpl.rcParams["axes.grid"] = True
+    mpl.rcParams["ytick.minor.visible"] = True
+    mpl.rcParams["xtick.minor.visible"] = True
+    mpl.rcParams["axes.grid.which"] = "both"
+
+    # Using old values to set both major and minor properties
+    mpl.rcParams["grid.color"] = "red"
+    mpl.rcParams["grid.linewidth"] = 1
+
+    # Overwrite some values for major and minor separately
+    mpl.rcParams["grid.major.color"] = "black"
+    mpl.rcParams["grid.major.linewidth"] = 2
+    mpl.rcParams["grid.minor.linestyle"] = ":"
+    mpl.rcParams["grid.minor.alpha"] = 0.6
+
+    plt.plot([0, 1], [0, 1])
+
+    plt.show()

lib/matplotlib/__init__.py

@@ -1309,11 +1309,18 @@ def is_interactive():
     return rcParams['interactive']
 
 
-def _val_or_rc(val, rc_name):
+def _val_or_rc(val, *rc_names):
     """
-    If *val* is None, return ``mpl.rcParams[rc_name]``, otherwise return val.
+    If *val* is None, the first not-None value in ``mpl.rcParams[rc_names[i]]``.
+    If all are None returns ``mpl.rcParams[rc_names[-1]]``.
     """
-    return val if val is not None else rcParams[rc_name]
+    if val is not None:
+        return val
+
+    for rc_name in rc_names[:-1]:
+        if rcParams[rc_name] is not None:
+            return rcParams[rc_name]
+    return rcParams[rc_names[-1]]
 
 
 def _init_tests():

lib/matplotlib/axis.py

@@ -125,16 +125,33 @@ def __init__(
                 zorder = mlines.Line2D.zorder
         self._zorder = zorder
 
-        grid_color = mpl._val_or_rc(grid_color, "grid.color")
-        grid_linestyle = mpl._val_or_rc(grid_linestyle, "grid.linestyle")
-        grid_linewidth = mpl._val_or_rc(grid_linewidth, "grid.linewidth")
+        grid_color = mpl._val_or_rc(
+            grid_color,
+            f"grid.{major_minor}.color",
+            "grid.color",
+        )
+        grid_linestyle = mpl._val_or_rc(
+            grid_linestyle,
+            f"grid.{major_minor}.linestyle",
+            "grid.linestyle",
+        )
+        grid_linewidth = mpl._val_or_rc(
+            grid_linewidth,
+            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.rcParams["grid.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",
+            )
+
         grid_kw = {k[5:]: v for k, v in kwargs.items() if k != "rotation_mode"}
 
         self.tick1line = mlines.Line2D(

lib/matplotlib/mpl-data/matplotlibrc

@@ -543,6 +543,16 @@
 #grid.linewidth: 0.8        # in points
 #grid.alpha:     1.0        # transparency, between 0.0 and 1.0
 
+#grid.major.color:     None # If None defaults to grid.color
+#grid.major.linestyle: None # If None defaults to grid.linestyle
+#grid.major.linewidth: None # If None defaults to grid.linewidth
+#grid.major.alpha:     None # If None defaults to grid.alpha
+
+#grid.minor.color:     None # If None defaults to grid.color
+#grid.minor.linestyle: None # If None defaults to grid.linestyle
+#grid.minor.linewidth: None # If None defaults to grid.linewidth
+#grid.minor.alpha:     None # If None defaults to grid.alpha
+
 
 ## ***************************************************************************
 ## * LEGEND                                                                  *

lib/matplotlib/rcsetup.py

@@ -361,6 +361,12 @@ def validate_color(s):
     raise ValueError(f'{s!r} does not look like a color arg')
 
 
+def _validate_color_or_None(s):
+    if s is None or cbook._str_equal(s, "None"):
+        return None
+    return validate_color(s)
+
+
 validate_colorlist = _listify_validator(
     validate_color, allow_stringlist=True, doc='return a list of colorspecs')
 
@@ -515,6 +521,13 @@ def _is_iterable_not_string_like(x):
     raise ValueError(f"linestyle {ls!r} is not a valid on-off ink sequence.")
 
 
+def _validate_linestyle_or_None(s):
+    if s is None or cbook._str_equal(s, "None"):
+        return None
+
+    return _validate_linestyle(s)
+
+
 validate_fillstyle = ValidateInStrings(
     'markers.fillstyle', ['full', 'left', 'right', 'bottom', 'top', 'none'])
 
@@ -1242,6 +1255,16 @@ def _convert_validator_spec(key, conv):
     "grid.linewidth":    validate_float,     # in points
     "grid.alpha":        validate_float,
 
+    "grid.major.color":        _validate_color_or_None,  # grid color
+    "grid.major.linestyle":    _validate_linestyle_or_None,  # solid
+    "grid.major.linewidth":    validate_float_or_None,     # in points
+    "grid.major.alpha":        validate_float_or_None,
+
+    "grid.minor.color":        _validate_color_or_None,  # grid color
+    "grid.minor.linestyle":    _validate_linestyle_or_None,  # solid
+    "grid.minor.linewidth":    validate_float_or_None,     # in points
+    "grid.minor.alpha":        validate_float_or_None,
+
     ## figure props
     # figure title
     "figure.titlesize":   validate_fontsize,

lib/matplotlib/rcsetup.pyi

@@ -48,6 +48,7 @@ def validate_color_or_auto(s: Any) -> ColorType | Literal["auto"]: ...
 def _validate_color_or_edge(s: Any) -> ColorType | Literal["edge"]: ...
 def validate_color_for_prop_cycle(s: Any) -> ColorType: ...
 def validate_color(s: Any) -> ColorType: ...
+def _validate_color_or_None(s: Any) -> ColorType | None: ...
 def validate_colorlist(s: Any) -> list[ColorType]: ...
 def _validate_color_or_linecolor(
     s: Any,
@@ -137,6 +138,7 @@ def validate_fillstylelist(
 ) -> list[Literal["full", "left", "right", "bottom", "top", "none"]]: ...
 def validate_markevery(s: Any) -> MarkEveryType: ...
 def _validate_linestyle(s: Any) -> LineStyleType: ...
+def _validate_linestyle_or_None(s: Any) -> LineStyleType | None: ...
 def validate_markeverylist(s: Any) -> list[MarkEveryType]: ...
 def validate_bbox(s: Any) -> Literal["tight", "standard"] | None: ...
 def validate_sketch(s: Any) -> None | tuple[float, float, float]: ...

lib/matplotlib/tests/test_axis.py

@@ -67,3 +67,31 @@ def test_get_tick_position_tick_params():
                    right=True, labelright=True, left=False, labelleft=False)
     assert ax.xaxis.get_ticks_position() == "top"
     assert ax.yaxis.get_ticks_position() == "right"
+
+
+def test_grid_rcparams():
+    """Tests that `grid.major/minor.*` overwrites `grid.*` in rcParams."""
+    plt.rcParams.update({
+        "axes.grid": True, "axes.grid.which": "both",
+        "ytick.minor.visible": True, "xtick.minor.visible": True,
+    })
+    def_linewidth = plt.rcParams["grid.linewidth"]
+    def_linestyle = plt.rcParams["grid.linestyle"]
+    def_alpha = plt.rcParams["grid.alpha"]
+
+    plt.rcParams.update({
+        "grid.color": "gray", "grid.minor.color": "red",
+        "grid.major.linestyle": ":", "grid.major.linewidth": 2,
+        "grid.minor.alpha": 0.6,
+    })
+    _, ax = plt.subplots()
+    ax.plot([0, 1])
+
+    assert ax.xaxis.get_major_ticks()[0].gridline.get_color() == "gray"
+    assert ax.xaxis.get_minor_ticks()[0].gridline.get_color() == "red"
+    assert ax.xaxis.get_major_ticks()[0].gridline.get_linewidth() == 2
+    assert ax.xaxis.get_minor_ticks()[0].gridline.get_linewidth() == def_linewidth
+    assert ax.xaxis.get_major_ticks()[0].gridline.get_linestyle() == ":"
+    assert ax.xaxis.get_minor_ticks()[0].gridline.get_linestyle() == def_linestyle
+    assert ax.xaxis.get_major_ticks()[0].gridline.get_alpha() == def_alpha
+    assert ax.xaxis.get_minor_ticks()[0].gridline.get_alpha() == 0.6