Fix: Do not use numeric tolerances for axline special cases

vertical lines (infinite slope) and two identical points as input need special handling in AxLine. The detection was using numeric tolerances, which lead to false-positive detection in cases that are close to but not exactly those special cases.

This PR removes the tolerances. The argument is the same as for the similar case matplotlib#28386 (comment)

Closes 28870.

Before, we only issued a warning. But we clearly cannot fulfill the intention
of a polar plot and therefore we should error out instead of just warn.

Also document that `polar()` should typically be called first to prevent having
a non-polar Axes already created.

Author: timhoffm

lib/matplotlib/lines.py

@@ -1506,8 +1506,8 @@ def get_transform(self):
                 points_transform.transform([self._xy1, self._xy2])
             dx = x2 - x1
             dy = y2 - y1
-            if np.allclose(x1, x2):
-                if np.allclose(y1, y2):
+            if dx == 0:
+                if dy == 0:
                     raise ValueError(
                         f"Cannot draw a line through two identical points "
                         f"(x={(x1, x2)}, y={(y1, y2)})")