This came up in dask, I wanted to confirm whether or not it's intentional before dask mirrors pandas: ```python In [6]: import pandas as pd In [7]: pd.concat([pd.Series([1, 2], name='foo')], axis='columns') Out[7]: foo 0 1 1 2 In [8]: type(_) Out[8]: pandas.core.frame.DataFrame ``` I think it's fine for `concat(..., axis=1)` to always return a DataFrame, but we should document that.