title: Why does knitr show a warning using auto.arima?
body: <p>It appears that running <code>auto.arima</code> from the <code>forecast</code> package in a <code>knitr</code> script always generates a warning - and I don't get this warning when I run it in normal R.</p>
  
  <p><strong>knitr Markdown Example Code:</strong></p>
  
  <pre><code>```{r}
  library(forecast)
  ```
  Spurious warning from forecast and knitr
  ========================================
  
  The following generates a warning that I don't think is valid
  
  ```{r}
  summary(auto.arima(WWWusage))
  ```
  </code></pre>
  
  <p>Produces this:</p>
  
  <p><img src="https://i.stack.imgur.com/MsEvF.gif" alt="knitroutput"></p>
  
  <p>Whereas running the following in R normally produces no such warning:</p>
  
  <pre><code>&gt; library(forecast)
  This is forecast 4.02 
  
  &gt; summary(auto.arima(WWWusage))
  Series: WWWusage 
  ARIMA(1,1,1)                    
  
  Coefficients:
           ar1     ma1
        0.6504  0.5256
  s.e.  0.0842  0.0896
  
  sigma^2 estimated as 9.793:  log likelihood=-254.15
  AIC=514.3   AICc=514.55   BIC=522.08
  
  Training set error measures:
         ME      RMSE       MAE       MPE      MAPE      MASE 
  0.3035616 3.1137542 2.4052748 0.2805566 1.9174634 0.5315228 
  </code></pre>
  
  <p>Also since this dataset is the example dataset for <code>auto.arima</code> I'm inclined to believe that the warning is incorrect (since I suspect a "good" example would be given).</p>
  
  <p>Any idea what is going on?</p>