Added option for an offset for MultipleLocator

Co-authored-by: Ruth Comer [email protected]

Author: jondoesntgit

doc/users/next_whats_new/multiplelocator_offset.rst

@@ -0,0 +1,17 @@
+``offset`` parameter for MultipleLocator
+----------------------------------------
+
+An *offset* may now be specified to shift all the ticks by the given value.
+
+.. plot::
+    :include-source: true
+
+    import matplotlib.pyplot as plt
+    import matplotlib.ticker as mticker
+
+    _, ax = plt.subplots()
+    ax.plot(range(10))
+    locator = mticker.MultipleLocator(base=3, offset=0.3)
+    ax.xaxis.set_major_locator(locator)
+
+    plt.show()

galleries/examples/ticks/tick-locators.py

@@ -37,8 +37,8 @@ def setup(ax, title):
 axs[0].xaxis.set_minor_locator(ticker.NullLocator())
 
 # Multiple Locator
-setup(axs[1], title="MultipleLocator(0.5)")
-axs[1].xaxis.set_major_locator(ticker.MultipleLocator(0.5))
+setup(axs[1], title="MultipleLocator(0.5, offset=0.2)")
+axs[1].xaxis.set_major_locator(ticker.MultipleLocator(0.5, offset=0.2))
 axs[1].xaxis.set_minor_locator(ticker.MultipleLocator(0.1))
 
 # Fixed Locator
@@ -53,7 +53,7 @@ def setup(ax, title):
 
 # Index Locator
 setup(axs[4], title="IndexLocator(base=0.5, offset=0.25)")
-axs[4].plot(range(0, 5), [0]*5, color='white')
+axs[4].plot([0]*5, color='white')
 axs[4].xaxis.set_major_locator(ticker.IndexLocator(base=0.5, offset=0.25))
 
 # Auto Locator

lib/matplotlib/tests/test_ticker.py

@@ -63,6 +63,12 @@ def test_basic(self):
                                9.441, 12.588])
         assert_almost_equal(loc.tick_values(-7, 10), test_value)
 
+    def test_basic_with_offset(self):
+        loc = mticker.MultipleLocator(base=3.147, offset=1.2)
+        test_value = np.array([-8.241, -5.094, -1.947, 1.2, 4.347, 7.494,
+                               10.641])
+        assert_almost_equal(loc.tick_values(-7, 10), test_value)
+
     def test_view_limits(self):
         """
         Test basic behavior of view limits.
@@ -80,6 +86,15 @@ def test_view_limits_round_numbers(self):
             loc = mticker.MultipleLocator(base=3.147)
             assert_almost_equal(loc.view_limits(-4, 4), (-6.294, 6.294))
 
+    def test_view_limits_round_numbers_with_offset(self):
+        """
+        Test that everything works properly with 'round_numbers' for auto
+        limit.
+        """
+        with mpl.rc_context({'axes.autolimit_mode': 'round_numbers'}):
+            loc = mticker.MultipleLocator(base=3.147, offset=1.3)
+            assert_almost_equal(loc.view_limits(-4, 4), (-4.994, 4.447))
+
     def test_set_params(self):
         """
         Create multiple locator with 0.7 base, and change it to something else.
@@ -88,6 +103,8 @@ def test_set_params(self):
         mult = mticker.MultipleLocator(base=0.7)
         mult.set_params(base=1.7)
         assert mult._edge.step == 1.7
+        mult.set_params(offset=3)
+        assert mult._offset == 3
 
 
 class TestAutoMinorLocator:

lib/matplotlib/ticker.py

@@ -1831,17 +1831,41 @@ def view_limits(self, vmin, vmax):
 
 class MultipleLocator(Locator):
     """
-    Set a tick on each integer multiple of the *base* within the view
-    interval.
+    Set a tick on each integer multiple of the *base* plus an *offset* within
+    the view interval.
     """
 
-    def __init__(self, base=1.0):
+    def __init__(self, base=1.0, offset=0.0):
+        """
+        Parameters
+        ----------
+        base : float > 0
+            Interval between ticks.
+        offset : float
+            Value added to each multiple of *base*.
+
+            .. versionadded:: 3.8
+        """
         self._edge = _Edge_integer(base, 0)
+        self._offset = offset
 
-    def set_params(self, base):
-        """Set parameters within this locator."""
+    def set_params(self, base=None, offset=None):
+        """
+        Set parameters within this locator.
+
+        Parameters
+        ----------
+        base : float > 0
+            Interval between ticks.
+        offset : float
+            Value added to each multiple of *base*.
+
+            .. versionadded:: 3.8
+        """
         if base is not None:
             self._edge = _Edge_integer(base, 0)
+        if offset is not None:
+            self._offset = offset
 
     def __call__(self):
         """Return the locations of the ticks."""
@@ -1852,19 +1876,20 @@ def tick_values(self, vmin, vmax):
         if vmax < vmin:
             vmin, vmax = vmax, vmin
         step = self._edge.step
+        vmin -= self._offset
+        vmax -= self._offset
         vmin = self._edge.ge(vmin) * step
         n = (vmax - vmin + 0.001 * step) // step
-        locs = vmin - step + np.arange(n + 3) * step
+        locs = vmin - step + np.arange(n + 3) * step + self._offset
         return self.raise_if_exceeds(locs)
 
     def view_limits(self, dmin, dmax):
         """
-        Set the view limits to the nearest multiples of *base* that
-        contain the data.
+        Set the view limits to the nearest tick values that contain the data.
         """
         if mpl.rcParams['axes.autolimit_mode'] == 'round_numbers':
-            vmin = self._edge.le(dmin) * self._edge.step
-            vmax = self._edge.ge(dmax) * self._edge.step
+            vmin = self._edge.le(dmin - self._offset) * self._edge.step + self._offset
+            vmax = self._edge.ge(dmax - self._offset) * self._edge.step + self._offset
             if vmin == vmax:
                 vmin -= 1
                 vmax += 1

lib/matplotlib/ticker.pyi

@@ -212,9 +212,8 @@ class LinearLocator(Locator):
     ) -> None: ...
 
 class MultipleLocator(Locator):
-    def __init__(self, base: float = ...) -> None: ...
-    # Makes set_params `base` argument mandatory
-    def set_params(self, base: float | None) -> None: ...  # type: ignore[override]
+    def __init__(self, base: float = ..., offset: float = ...) -> None: ...
+    def set_params(self, base: float | None = ..., offset: float | None = ...) -> None: ...
     def view_limits(self, dmin: float, dmax: float) -> tuple[float, float]: ...
 
 class _Edge_integer: