code: public static Column mean(DataRowsFacade dataFrame, String columnName) {
          return dataFrame.get().groupBy(columnName).agg(avg(columnName)).col(columnName);
      }
docs: Mean for a column
  
  @param dataFrame  the dataframe to
  get the column fron
  @param columnName the name of the column to get the mean for
  @return the column that represents the mean