Allow setting default AutoMinorLocator (matplotlib#18715)

Fixes matplotlib#14233


Co-authored-by: Vladimir <vladimir@igenius.net>
Co-authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>

Author: 3 people

doc/users/next_whats_new/auto_minor_tick.rst

@@ -0,0 +1,5 @@
+rcParams for ``AutoMinorLocator`` divisions
+-------------------------------------------
+The rcParams :rc:`xtick.minor.ndivs` and :rc:`ytick.minor.ndivs` have been
+added to enable setting the default number of divisions; if set to ``auto``,
+the number of divisions will be chosen by the distance between major ticks.

lib/matplotlib/mpl-data/matplotlibrc

@@ -489,6 +489,7 @@
 #xtick.major.bottom:  True    # draw x axis bottom major ticks
 #xtick.minor.top:     True    # draw x axis top minor ticks
 #xtick.minor.bottom:  True    # draw x axis bottom minor ticks
+#xtick.minor.ndivs:   auto    # number of minor ticks between the major ticks on x-axis
 #xtick.alignment:     center  # alignment of xticks
 
 #ytick.left:          True    # draw ticks on the left side
@@ -510,6 +511,7 @@
 #ytick.major.right:   True    # draw y axis right major ticks
 #ytick.minor.left:    True    # draw y axis left minor ticks
 #ytick.minor.right:   True    # draw y axis right minor ticks
+#ytick.minor.ndivs:   auto    # number of minor ticks between the major ticks on y-axis
 #ytick.alignment:     center_baseline  # alignment of yticks
 
 

lib/matplotlib/rcsetup.py

@@ -568,6 +568,14 @@ def _validate_greaterequal0_lessequal1(s):
         raise RuntimeError(f'Value must be >=0 and <=1; got {s}')
 
 
+def _validate_int_greaterequal0(s):
+    s = validate_int(s)
+    if s >= 0:
+        return s
+    else:
+        raise RuntimeError(f'Value must be >=0; got {s}')
+
+
 def validate_hatch(s):
     r"""
     Validate a hatch pattern.
@@ -587,6 +595,24 @@ def validate_hatch(s):
 validate_dashlist = _listify_validator(validate_floatlist)
 
 
+def _validate_minor_tick_ndivs(n):
+    """
+    Validate ndiv parameter related to the minor ticks.
+    It controls the number of minor ticks to be placed between
+    two major ticks.
+    """
+
+    if isinstance(n, str) and n.lower() == 'auto':
+        return n
+    try:
+        n = _validate_int_greaterequal0(n)
+        return n
+    except (RuntimeError, ValueError):
+        pass
+
+    raise ValueError("'tick.minor.ndivs' must be 'auto' or non-negative int")
+
+
 _prop_validators = {
         'color': _listify_validator(validate_color_for_prop_cycle,
                                     allow_stringlist=True),
@@ -1093,6 +1119,8 @@ def _convert_validator_spec(key, conv):
     "xtick.minor.bottom":  validate_bool,      # draw bottom minor xticks
     "xtick.major.top":     validate_bool,      # draw top major xticks
     "xtick.major.bottom":  validate_bool,      # draw bottom major xticks
+    # number of minor xticks
+    "xtick.minor.ndivs":   _validate_minor_tick_ndivs,
     "xtick.labelsize":     validate_fontsize,  # fontsize of xtick labels
     "xtick.direction":     ["out", "in", "inout"],  # direction of xticks
     "xtick.alignment":     ["center", "right", "left"],
@@ -1114,6 +1142,8 @@ def _convert_validator_spec(key, conv):
     "ytick.minor.right":   validate_bool,      # draw right minor yticks
     "ytick.major.left":    validate_bool,      # draw left major yticks
     "ytick.major.right":   validate_bool,      # draw right major yticks
+    # number of minor yticks
+    "ytick.minor.ndivs":   _validate_minor_tick_ndivs,
     "ytick.labelsize":     validate_fontsize,  # fontsize of ytick labels
     "ytick.direction":     ["out", "in", "inout"],  # direction of yticks
     "ytick.alignment":     [

lib/matplotlib/tests/test_ticker.py

@@ -211,6 +211,60 @@ def test_additional(self, lim, ref):
 
         assert_almost_equal(ax.yaxis.get_ticklocs(minor=True), ref)
 
+    @pytest.mark.parametrize('use_rcparam', [False, True])
+    @pytest.mark.parametrize(
+        'lim, ref', [
+            ((0, 1.39),
+             [0.05, 0.1, 0.15, 0.25, 0.3, 0.35, 0.45, 0.5, 0.55, 0.65, 0.7,
+              0.75, 0.85, 0.9, 0.95, 1.05, 1.1, 1.15, 1.25, 1.3, 1.35]),
+            ((0, 0.139),
+             [0.005, 0.01, 0.015, 0.025, 0.03, 0.035, 0.045, 0.05, 0.055,
+              0.065, 0.07, 0.075, 0.085, 0.09, 0.095, 0.105, 0.11, 0.115,
+              0.125, 0.13, 0.135]),
+        ])
+    def test_number_of_minor_ticks_auto(self, lim, ref, use_rcparam):
+        if use_rcparam:
+            context = {'xtick.minor.ndivs': 'auto', 'ytick.minor.ndivs': 'auto'}
+            kwargs = {}
+        else:
+            context = {}
+            kwargs = {'n': 'auto'}
+
+        with mpl.rc_context(context):
+            fig, ax = plt.subplots()
+            ax.set_xlim(*lim)
+            ax.set_ylim(*lim)
+            ax.xaxis.set_minor_locator(mticker.AutoMinorLocator(**kwargs))
+            ax.yaxis.set_minor_locator(mticker.AutoMinorLocator(**kwargs))
+            assert_almost_equal(ax.xaxis.get_ticklocs(minor=True), ref)
+            assert_almost_equal(ax.yaxis.get_ticklocs(minor=True), ref)
+
+    @pytest.mark.parametrize('use_rcparam', [False, True])
+    @pytest.mark.parametrize(
+        'n, lim, ref', [
+            (2, (0, 4), [0.5, 1.5, 2.5, 3.5]),
+            (4, (0, 2), [0.25, 0.5, 0.75, 1.25, 1.5, 1.75]),
+            (10, (0, 1), [0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9]),
+        ])
+    def test_number_of_minor_ticks_int(self, n, lim, ref, use_rcparam):
+        if use_rcparam:
+            context = {'xtick.minor.ndivs': n, 'ytick.minor.ndivs': n}
+            kwargs = {}
+        else:
+            context = {}
+            kwargs = {'n': n}
+
+        with mpl.rc_context(context):
+            fig, ax = plt.subplots()
+            ax.set_xlim(*lim)
+            ax.set_ylim(*lim)
+            ax.xaxis.set_major_locator(mticker.MultipleLocator(1))
+            ax.xaxis.set_minor_locator(mticker.AutoMinorLocator(**kwargs))
+            ax.yaxis.set_major_locator(mticker.MultipleLocator(1))
+            ax.yaxis.set_minor_locator(mticker.AutoMinorLocator(**kwargs))
+            assert_almost_equal(ax.xaxis.get_ticklocs(minor=True), ref)
+            assert_almost_equal(ax.yaxis.get_ticklocs(minor=True), ref)
+
 
 class TestLogLocator:
     def test_basic(self):

lib/matplotlib/ticker.py

@@ -2869,7 +2869,11 @@ def __init__(self, n=None):
         major ticks; e.g., n=2 will place a single minor tick midway
         between major ticks.
 
-        If *n* is omitted or None, it will be set to 5 or 4.
+        If *n* is omitted or None, the value stored in rcParams will be used.
+        In case *n* is set to 'auto', it will be set to 4 or 5. If the distance
+        between the major ticks equals 1, 2.5, 5 or 10 it can be perfectly
+        divided in 5 equidistant sub-intervals with a length multiple of
+        0.05. Otherwise it is divided in 4 sub-intervals.
         """
         self.ndivs = n
 
@@ -2892,6 +2896,14 @@ def __call__(self):
 
         if self.ndivs is None:
 
+            if self.axis.axis_name == 'y':
+                self.ndivs = mpl.rcParams['ytick.minor.ndivs']
+            else:
+                # for x and z axis
+                self.ndivs = mpl.rcParams['xtick.minor.ndivs']
+
+        if self.ndivs == 'auto':
+
             majorstep_no_exponent = 10 ** (np.log10(majorstep) % 1)
 
             if np.isclose(majorstep_no_exponent, [1.0, 2.5, 5.0, 10.0]).any():