code: def true_range(arg, high_col='high', low_col='low', close_col='close', skipna=0):
      
      _ensure_col(arg, high_col=high_col, low_col=low_col, close_col=close_col)
      yclose = arg[close_col].shift(1)
      low, high = arg[low_col], arg[high_col]
      mx = pd.DataFrame({'a': high, 'b': yclose}).max(axis=1, skipna=skipna)
      mn = pd.DataFrame({'a': low, 'b': yclose}).min(axis=1, skipna=skipna)
      result = mx - mn
      return pd.Series(result, index=arg.index, name='true_range')
docs: http://en.wikipedia.org/wiki/Average_true_range
      The greatest of the following:
      - Current High less the current Low
      - Current High less the previous Close (absolute value)
      - Curre    nt Low less the previous Close (absolute value)