title: choose colour when plotting multivariate time series with ggfortify as overlay
body: <pre><code>library(ggfortify)
  </code></pre>
  
  <p>With ggfortify, if I plot one time series, I can set the line colour as follows:</p>
  
  <pre><code>autoplot(myts1,ts.colour='blue')
  </code></pre>
  
  <p>I can plot two ts objects in one graph:</p>
  
  <pre><code>autoplot(cbind(myts1,myts2),facets=FALSE)
  </code></pre>
  
  <p>But how can I set for example the line colour for the first ts 'blue' and for the second 'red'? In the second example, ts.colour doesn't work at all.</p>
  
  <p>edit: here is a working example</p>
  
  <pre><code>myts1 = filter(rnorm(100), filter=rep(1,20),circular=TRUE)
  myts2 = sin(seq(0,20,length.out=100))*5+5
  autoplot(cbind(myts1,myts2),facets=FALSE)
  </code></pre>