feat: add dunder method for math operations.

Author: paquiteau

lib/mpl_toolkits/axes_grid1/axes_size.py

@@ -7,6 +7,10 @@ class (or others) to determine the size of each Axes. The unit
 Note that this class is nothing more than a simple tuple of two
 floats. Take a look at the Divider class to see how these two
 values are used.
+
+Once created, the unit classes can be modified by simple arithmetic
+operations: addition /subtraction with another unit type or a real number and scaling
+(multiplication or division) by a real number.
 """
 
 from numbers import Real
@@ -17,14 +21,33 @@ class (or others) to determine the size of each Axes. The unit
 
 class _Base:
     def __rmul__(self, other):
+        return self * other
+
+    def __mul__(self, other):
+        if not isinstance(other, Real):
+            return NotImplemented
         return Fraction(other, self)
 
+    def __div__(self, other):
+        return (1 / other) * self
+
     def __add__(self, other):
         if isinstance(other, _Base):
             return Add(self, other)
         else:
             return Add(self, Fixed(other))
 
+    def __neg__(self):
+        return -1 * self
+
+    def __radd__(self, other):
+        # other cannot be a _Base instance, because A + B would trigger
+        # A.__add__(B) first.
+        return Add(self, Fixed(other))
+
+    def __sub__(self, other):
+        return self + (-other)
+
     def get_size(self, renderer):
         """
         Return two-float tuple with relative and absolute sizes.