Warn if different axis projection requested

lib/matplotlib/figure.py

@@ -1485,6 +1485,10 @@ def gca(self, **kwargs):
                 # continue and a new axes will be created
                 if key == ckey and isinstance(cax, projection_class):
                     return cax
+                else:
+                    warnings.warn('Requested projection is different from '
+                                  'current axis projection, creating new axis '
+                                  'with requested projection.')
 
         # no axes found, so create one which spans the figure
         return self.add_subplot(1, 1, 1, **kwargs)

lib/matplotlib/pyplot.py

@@ -2279,6 +2279,11 @@ def polar(*args, **kwargs):
     strings, as in :func:`~matplotlib.pyplot.plot`.
 
     """
+    # If an axis already exists, check if it has a polar projection
+    if gcf().get_axes():
+        if not isinstance(gca(), PolarAxes):
+            warnings.warn('Trying to create polar plot on an axis that does '
+                          'not have a polar projection.')
     ax = gca(polar=True)
     ret = ax.plot(*args, **kwargs)
     return ret