oops; failed to commit after fixing this little bugaboo; should have …

…been len(y) instead of len(x)

lib/matplotlib/axes.py

@@ -3626,7 +3626,7 @@ def hlines(self, y, xmin, xmax, colors='k', linestyles='solid',
         self.add_collection(coll)
         coll.update(kwargs)
 
-        if len(x) > 0:
+        if len(y) > 0:
             minx = min(xmin.min(), xmax.min())
             maxx = max(xmin.max(), xmax.max())
             miny = y.min()