Fix error.

Author: dcherian

xarray/plot/plot.py

@@ -204,7 +204,7 @@ def line(darray, *args, **kwargs):
 
     ax = get_axis(figsize, size, aspect, ax)
 
-    error_msg = 'must be either None or %r' % (' or '.join(darray.dims))
+    error_msg = 'must be either None or one of %r' % list(darray.dims)
 
     if x not in [None, *darray.dims]:
         raise ValueError('x ' + error_msg)