Change skew() angle definitions.

Instead of specifying the angles as rotations of the axes, it is easier
(and more standard) to think of the angles in the skew transform as
shearing angles along each axis. Doing so means the X and Y angles
are swapped from how they were used before, so this commit also
updates the example.

Author: dopplershift

examples/api/skewt.py

@@ -131,7 +131,7 @@ def _set_lim_and_transforms(self):
         # We keep the pre-transAxes transform around for other users, like the
         # spines for finding bounds
         self.transDataToAxes = self.transScale + (self.transLimits +
-                transforms.Affine2D().skew_deg(0, rot))
+                transforms.Affine2D().skew_deg(rot, 0))
 
         # Create the full transform from Data to Pixels
         self.transData = self.transDataToAxes + self.transAxes
@@ -141,7 +141,7 @@ def _set_lim_and_transforms(self):
         self._xaxis_transform = (transforms.blended_transform_factory(
                     self.transScale + self.transLimits,
                     transforms.IdentityTransform()) +
-                transforms.Affine2D().skew_deg(0, rot)) + self.transAxes
+                transforms.Affine2D().skew_deg(rot, 0)) + self.transAxes
 
 # Now register the projection with matplotlib so the user can select
 # it.

lib/matplotlib/transforms.py

@@ -1876,38 +1876,38 @@ def scale(self, sx, sy=None):
         self.invalidate()
         return self
 
-    def skew(self, xAxisRot, yAxisRot):
+    def skew(self, xShear, yShear):
         """
         Adds a skew in place.
 
-        *xAxisRot* and *yAxisRot* are the rotations of the *x*- and
+        *xShear* and *yShear* are the shear angles along the *x*- and
         *y*-axes, respectively, in radians.
 
         Returns *self*, so this method can easily be chained with more
         calls to :meth:`rotate`, :meth:`rotate_deg`, :meth:`translate`
         and :meth:`scale`.
         """
-        rotX = np.tan(xAxisRot)
-        rotY = np.tan(yAxisRot)
+        rotX = np.tan(xShear)
+        rotY = np.tan(yShear)
         skew_mtx = np.array(
-                [[1.0, rotY, 0.0], [rotX, 1.0, 0.0], [0.0, 0.0, 1.0]],
+                [[1.0, rotX, 0.0], [rotY, 1.0, 0.0], [0.0, 0.0, 1.0]],
                 np.float_)
         self._mtx = np.dot(skew_mtx, self._mtx)
         self.invalidate()
         return self
 
-    def skew_deg(self, xAxisRot, yAxisRot):
+    def skew_deg(self, xShear, yShear):
         """
         Adds a skew in place.
 
-        *xAxisRot* and *yAxisRot* are the rotations of the *x*- and
+        *xShear* and *yShear* are the shear angles along the *x*- and
         *y*-axes, respectively, in degrees.
 
         Returns *self*, so this method can easily be chained with more
         calls to :meth:`rotate`, :meth:`rotate_deg`, :meth:`translate`
         and :meth:`scale`.
         """
-        return self.skew(np.deg2rad(xAxisRot), np.deg2rad(yAxisRot))
+        return self.skew(np.deg2rad(xShear), np.deg2rad(yShear))
 
     def _get_is_separable(self):
         mtx = self.get_matrix()