Move axisartist towards using standard Transforms.

axisartist can generate "slanted" or "curved" axes defined using either
a custom Transform, or a custom pair of callables (forward, backward)
which define the Transform.  Instead of repeatedly testing the two cases
in `transform_xy` and `inv_transform_xy`, just wrap the
pair-of-callables into a custom Transform subclass (which should
probably not be moved to the main library, as it uses the transposed
convention compared to the usual one), and expose it through a getter.

This allows later combining this transform using "standard" transform
addition (and get benefits such as single multiplication of matrices
in affine transforms, exact cancellation of inverses, or (later) more
accurate transforming of arcs in polar transforms), instead of having to
manually call successive transforms.  See e.g. the two changes in the
local transform_xy definitions.

Author: anntzer

lib/mpl_toolkits/axisartist/floating_axes.py

@@ -74,9 +74,8 @@ def get_tick_iterators(self, axes):
         ymin, ymax = sorted(extremes[2:])
 
         def transform_xy(x, y):
-            x1, y1 = grid_finder.transform_xy(x, y)
-            x2, y2 = axes.transData.transform(np.array([x1, y1]).T).T
-            return x2, y2
+            trf = grid_finder.get_transform() + axes.transData
+            return trf.transform(np.column_stack([x, y])).T
 
         if self.nth_coord == 0:
             mask = (ymin <= yy0) & (yy0 <= ymax)

lib/mpl_toolkits/axisartist/grid_finder.py

@@ -92,6 +92,34 @@ def _add_pad(self, x_min, x_max, y_min, y_max):
         return x_min - dx, x_max + dx, y_min - dy, y_max + dy
 
 
+class _User2DTransform(Transform):
+    """A transform defined by two user-set functions."""
+
+    input_dims = output_dims = 2
+
+    def __init__(self, forward, backward):
+        """
+        Parameters
+        ----------
+        forward, backward : callable
+            The forward and backward transforms, as taking ``x`` and ``y`` as
+            separate arguments and returning ``(tr_x, tr_y)``.
+        """
+        # The normal Matplotlib convention would be to take and return an
+        # (N, 2) array but axisartist uses the transposed version.
+        super().__init__()
+        self._forward = forward
+        self._backward = backward
+
+    def transform_non_affine(self, values):
+        # docstring inherited
+        return np.transpose(self._forward(*np.transpose(values)))
+
+    def inverted(self):
+        # docstring inherited
+        return type(self)(self._backward, self._forward)
+
+
 class GridFinder:
     def __init__(self,
                  transform,
@@ -123,7 +151,7 @@ def __init__(self,
         self.grid_locator2 = grid_locator2
         self.tick_formatter1 = tick_formatter1
         self.tick_formatter2 = tick_formatter2
-        self.update_transform(transform)
+        self.set_transform(transform)
 
     def get_grid_info(self, x1, y1, x2, y2):
         """
@@ -214,27 +242,26 @@ def _clip_grid_lines_and_find_ticks(self, lines, values, levs, bb):
 
         return gi
 
-    def update_transform(self, aux_trans):
-        if not isinstance(aux_trans, Transform) and len(aux_trans) != 2:
-            raise TypeError("'aux_trans' must be either a Transform instance "
-                            "or a pair of callables")
-        self._aux_transform = aux_trans
+    def set_transform(self, aux_trans):
+        if isinstance(aux_trans, Transform):
+            self._aux_transform = aux_trans
+        elif len(aux_trans) == 2 and all(map(callable, aux_trans)):
+            self._aux_transform = _User2DTransform(*aux_trans)
+        else:
+            raise TypeError("'aux_trans' must be either a Transform "
+                            "instance or a pair of callables")
+
+    def get_transform(self):
+        return self._aux_transform
+
+    update_transform = set_transform  # backcompat alias.
 
     def transform_xy(self, x, y):
-        aux_trf = self._aux_transform
-        if isinstance(aux_trf, Transform):
-            return aux_trf.transform(np.column_stack([x, y])).T
-        else:
-            transform_xy, inv_transform_xy = aux_trf
-            return transform_xy(x, y)
+        return self._aux_transform.transform(np.column_stack([x, y])).T
 
     def inv_transform_xy(self, x, y):
-        aux_trf = self._aux_transform
-        if isinstance(aux_trf, Transform):
-            return aux_trf.inverted().transform(np.column_stack([x, y])).T
-        else:
-            transform_xy, inv_transform_xy = aux_trf
-            return inv_transform_xy(x, y)
+        return self._aux_transform.inverted().transform(
+            np.column_stack([x, y])).T
 
     def update(self, **kw):
         for k in kw:

lib/mpl_toolkits/axisartist/grid_helper_curvelinear.py

@@ -202,10 +202,8 @@ def get_tick_iterators(self, axes):
             xx0 = xx0[mask]
 
         def transform_xy(x, y):
-            x1, y1 = grid_finder.transform_xy(x, y)
-            x2y2 = axes.transData.transform(np.array([x1, y1]).transpose())
-            x2, y2 = x2y2.transpose()
-            return x2, y2
+            trf = grid_finder.get_transform() + axes.transData
+            return trf.transform(np.column_stack([x, y])).T
 
         # find angles
         if self.nth_coord == 0: