title: How to detect seasonality using acf and pacf
body: <p>I am trying to find out if a time series is seasonal and if it is, what is the seasonal pattern.
  I have two questions:</p>
  <ol>
  <li>Should I plot acf/pacf using the detrended time series? (I do STL decomposition and select the seasonal component to plot acf/pacf). Is this the correct way to plot them?</li>
  </ol>
  <pre><code>from statsmodels.tsa.seasonal import STL
  from statsmodels.graphics.tsaplots import plot_acf, plot_pacf
  
  res = STL(data).fit()
  plot_acf(res.seasonal)
  plot_pacf(res.seasonal)
  </code></pre>
  <ol start="2">
  <li>If it is, can you explain the seasonality in these two data sets below?
  <a href="https://i.stack.imgur.com/nbg3S.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nbg3S.png" alt="acf/pacf 1" /></a>
  <a href="https://i.stack.imgur.com/b1vcj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/b1vcj.png" alt="acf/pacf 2" /></a></li>
  </ol>