title: how to get last column of pandas series
body: <p>I am trying to count frequencies of an array.
  I've read this <a href="https://stackoverflow.com/questions/40144769/how-to-select-the-last-column-of-dataframe">post</a>, I am using DataFrame and get a series.</p>
  
  <pre><code>&gt;&gt;&gt; a = np.array([1, 1, 5, 0, 1, 2, 2, 0, 1, 4])
  &gt;&gt;&gt; df = pd.DataFrame(a, columns=['a'])
  &gt;&gt;&gt; b = df.groupby('a').size()
  &gt;&gt;&gt; b
  a
  0    2
  1    4
  2    2
  4    1
  5    1
  dtype: int64
  &gt;&gt;&gt; b.iloc[:,-1]
  </code></pre>
  
  <p>when i try to get the last column, i got this error.</p>
  
  <pre><code>Traceback (most recent call last):   File "&lt;stdin&gt;", line 1, in &lt;module&gt;   File "/Users/pan/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 1472, in __getitem__
      return self._getitem_tuple(key)   File "/Users/pan/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 2013, in _getitem_tuple
      self._has_valid_tuple(tup)   File "/Users/pan/anaconda3/lib/python3.6/site-packages/pandas/core/indexing.py", line 220, in _has_valid_tuple
      raise IndexingError('Too many indexers') pandas.core.indexing.IndexingError: Too many indexers
  </code></pre>
  
  <p>how to get the last column of <code>b</code>?</p>