id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
237325677,MDU6SXNzdWUyMzczMjU2Nzc=,1461,Bug: Faceting fails with levels  (easy to fix),15937254,closed,0,,,2,2017-06-20T19:52:30Z,2018-04-02T21:26:17Z,2018-03-06T05:05:59Z,NONE,,,,"Faceting  example fails with levels
```Python
flatui = [""#9b59b6"", ""#3498db"", ""#95a5a6"", ""#e74c3c"", ""#34495e"", ""#2ecc71""]
g_simple = t.plot(x='lon', y='lat', col='time', col_wrap=3,
                  levels=[0, 12, 18, 30], colors=flatui)
```
The generated error is
> ValueError: Can't specify both cmap and colors.

I tracked this down inside newplotfunc  to the line 
```Python
 return _easy_facetgrid(**allargs)
```
which calls `FacetGrid.map_dataarray`

This function does not check if levels is assigned. And hence automatically adds a cmap on the second panel of the plot
```Python
        cmap_args = inspect.getargspec(_determine_cmap_params).args
        cmap_kwargs.update((a, kwargs[a]) for a in cmap_args if a in kwargs)

        cmap_params = _determine_cmap_params(**cmap_kwargs)
```

the check fails after that. 

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