title: Low pass Linear Filter in python
body: <p>I have red a lot about filtering time series but I really have difficulties to understand properly the scipy <code>scipy.signal.filtfilt</code>.</p>
  <p>In particular its parameters a and b.
  For example how I should reproduce the following filter?
  Even with others libs if is easier.</p>
  <p><a href="https://i.stack.imgur.com/10wwF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/10wwF.png" alt="enter image description here" /></a></p>
  <p>Can anyone help me? I give you a starting point:</p>
  <pre><code>np.random.seed(123)
  N = 100
  rng = pd.date_range('2019-01-01', freq='min', periods=N)
  df = pd.DataFrame(np.random.rand(N, 1), index=rng)
  
  </code></pre>