title: Multiple time-series with face_wrap
body: <p>I am working on a time-series project. I would like to compare the "y" data to the "y_hat". What I call "y" is the data from my dataset and "y_hat" is what my algorithms have predicted.  </p>
  
  <p>I have tried using facet_wrap, unfortunately it plots one time-series by category as you can see on the image :</p>
  
  <p><a href="https://i.stack.imgur.com/6N4Nm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/6N4Nm.png" alt="enter image description here"></a></p>
  
  <pre><code>nb_of_algorithm &lt;- 6
  gather_df &lt;- df_all %&gt;% gather(key="Algorithm", "values", 2:nb_of_algorithm)
  
  ggplot(gather_df, aes(x = ds, y = values)) +
          geom_line(aes(color = Algorithm)) +
          scale_color_brewer(palette="Dark2") +
          facet_wrap(~ Algorithm)
  </code></pre>
  
  <p>And I add a sample of what my dataframe looks like</p>
  
  <pre><code>            ds     Algorithm    values
  1   2018-10-19             y  8115.000
  2   2018-10-20             y  8730.000
  3   2018-10-21             y  7155.000
  4   2018-10-22             y   570.000
  164 2018-10-19 y_hat_xgboost  3458.394
  165 2018-10-20 y_hat_xgboost  6424.176
  166 2018-10-21 y_hat_xgboost  3416.893
  167 2018-10-22 y_hat_xgboost 12041.853
  168 2018-10-23 y_hat_xgboost  9801.245
  169 2018-10-24 y_hat_xgboost 11081.888
  327 2018-10-19  y_hat_nnetar  7188.586
  328 2018-10-20  y_hat_nnetar  6606.201
  329 2018-10-21  y_hat_nnetar 10488.071
  330 2018-10-22  y_hat_nnetar 17417.546
  331 2018-10-23  y_hat_nnetar 14230.000
  </code></pre>
  
  <p>The expected results would be the same graph as above, with on the same plot:<br>
  *  "y" and "y_hat_xgboost"<br>
  * "y" and "y_hat_nnetar"<br>
  * and so on ...</p>
  
  <p>So I can compare them to the real data</p>
  
  <p>Thanks for your help</p>