Raise error if points are the same

dstansby · dstansby · commit ee7707b96f58 · 2019-02-24T11:37:46.000Z
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -957,6 +957,10 @@ def axline(self, xy1, xy2, **kwargs):
         x2, y2 = xy2
         # If x values the same, we have a vertical line
         if np.allclose(x1, x2):
+            if np.allclose(y1, y2):
+                raise ValueError(
+                    'Cannot draw a line through two identical points '
+                    f'(got x1={x1}, x2={x2}, y1={y1}, y2={y2}).')
             line = self.axvline(x1, **kwargs)
             return line
 
