title: Scale for colour is already present, overlay plots in ggplot2
body: <p>I'm attempting to overlay two plots with ggplot2. I don't have any problems doing it with <code>scale_x_manual</code> (as it's described in other <a href="https://stackoverflow.com/questions/15011422/ggplot2-using-two-different-color-scales-for-overlayed-plots">question</a>), but it crashes when I have to use <code>scale_color_gradient</code>...</p>
  
  <p>My code is: </p>
  
  <pre><code>ggplot() + 
    geom_point(data = mtcars, aes(x = mpg, y = qsec, color = qsec)) +
    scale_color_gradient(low = "gray", high = "blue") +
    geom_point(data = mtcars, aes(x = drat, y = wt, color = wt)) +
    scale_color_gradient(low = "gray", high = "red")
  </code></pre>
  
  <p>But it prompts this message and only keeps one scale: </p>
  
  <pre><code>Scale for 'colour' is already present. 
  Adding another scale for 'colour', which will replace the existing scale.
  </code></pre>
  
  <p>I've tried different options (<code>scale_color_manual</code>, <code>scale_fill_gradient</code>, <code>scale_color_gradient2</code> or <code>scale_color_gradientn</code>...) but nothing seems to work. Please, any suggestions? Thanks for your help!</p>