Raise a better error with row and col_wrap

Closes mwaskom#465

Author: mwaskom

seaborn/axisgrid.py

@@ -249,6 +249,9 @@ def __init__(self, data, row=None, col=None, hue=None, col_wrap=None,
 
         self._col_wrap = col_wrap
         if col_wrap is not None:
+            if row is not None:
+                err = "Cannot use `row` and `col_wrap` together."
+                raise ValueError(err)
             ncol = col_wrap
             nrow = int(np.ceil(len(data[col].unique()) / col_wrap))
         self._ncol = ncol

seaborn/tests/test_axisgrid.py

@@ -104,6 +104,9 @@ def test_col_wrap(self):
         nt.assert_equal(g_wrap._ncol, 4)
         nt.assert_equal(g_wrap._nrow, 3)
 
+        with nt.assert_raises(ValueError):
+            g = ag.FacetGrid(self.df, row="b", col="d", col_wrap=4)
+
         plt.close("all")
 
     def test_normal_axes(self):