regression: bar plot with multi-column category doesn't work anymore
#### Code Sample, a copy-pastable example if possible

Doing a bar plot using a multi-column category used to work
```python
df.plot.barh(x=['fruit', 'animal'], y='size')
```

In pandas 0.22.0:
<img width="575" alt="screen shot 2018-06-08 at 16 34 51" src="https://user-images.githubusercontent.com/44946/41164650-fa16b13e-6b3b-11e8-8586-1da29dca3688.png">

since pandas 0.23.0:
<img width="1113" alt="screen shot 2018-06-08 at 16 39 36" src="https://user-images.githubusercontent.com/44946/41164687-0a2b8004-6b3c-11e8-83ab-62e930ef643f.png">

Is it intended that this stopped working? 
I kind of liked the feature. 