geom_bar with stat='identity' does not stack values when only one group present

```python
d = {
'name': ['foo', 'foo', 'bar', 'bar'],
'g': ['A', 'B', 'A', 'B'],
'value': [80, 90, 15, 35]
}

ggplot(d) + geom_bar(aes(x='name', y='value', group='name'), stat='identity')
```
Actual result:

When values are assigned to their own group, everything works as expected (note the mapping `group='g'`):