title: Pandas: Sorting columns by their mean value
body: <p>I have a dataframe in Pandas, I would like to sort its columns (i.e. get a new dataframe, or a view) according to the mean value of its columns (or e.g. by their std value). The documentation talks about <a href="http://pandas.pydata.org/pandas-docs/stable/basics.html#sorting-by-index-and-value" rel="noreferrer">sorting by label or value</a>, but I could not find anything on custom sorting methods.</p>
  
  <p>How can I do this? </p>