title: FB Prophet changes the training data
body: <p>I am working on a time series prediction with a dataset that has this header</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>ds</th>
  <th>y</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>2018-01-31</td>
  <td>39.667406</td>
  </tr>
  <tr>
  <td>2018-02-28</td>
  <td>42.091133</td>
  </tr>
  <tr>
  <td>2018-03-31</td>
  <td>40.845842</td>
  </tr>
  <tr>
  <td>2018-04-30</td>
  <td>42.177686</td>
  </tr>
  <tr>
  <td>2018-05-31</td>
  <td>42.087838</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>and fit the data to the model and did some predictions with the following code</p>
  <pre><code>Model = Prophet()
  Model.fit(df_prophet_ventana)
  future = Model.make_future_dataframe(periods=4, freq='M')
  
  forecast = Model.predict(future)
  forecast[['ds', 'yhat', 'yhat_lower', 'yhat_upper']].tail()
  </code></pre>
  <p>The problem is when I look at the values in forecast</p>
  <pre><code>forecast[['ds','yhat']].head()
  </code></pre>
  <p>it return a dataset with different values of the original data</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>ds</th>
  <th>yhat</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>2018-01-31</td>
  <td>41.508584</td>
  </tr>
  <tr>
  <td>2018-02-28</td>
  <td>41.518272</td>
  </tr>
  <tr>
  <td>2018-03-31</td>
  <td>41.528999</td>
  </tr>
  <tr>
  <td>2018-04-30</td>
  <td>41.539380</td>
  </tr>
  <tr>
  <td>2018-05-31</td>
  <td>41.549945</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>Why does the values change?</p>