title: Mixing line and scatterplot in ggplot
body: <p>I've looked around a fair bit but I am stumped on this one. <strong>I cannot  find a way to plot a line that is unassociated with a scatterplot.</strong> Here is some of my data and code to clarify the problem. I have data of the following form</p>
  
  <pre><code>&gt; head(allData)
    AnnounceDate MarketProbability DealStatus binary BrierScore
  1   2000-04-10         0.3333333   Complete      1  0.2340565
  2   2000-06-14         0.2142857   Complete      1  0.3618200
  3   2000-06-26         0.6846154   Complete      1  0.3690167
  4   2000-06-16         0.1875000   Complete      1  0.4364041
  5   2000-10-05         0.9555556   Complete      1  0.3078432
  6   2000-10-19         0.8500000   Complete      1  0.2670799
  </code></pre>
  
  <p>I would like to create a scatter plot of the <code>MarketProbabilities</code> vs. the <code>AnnounceDate</code>, and identify whether a <code>DealStatus</code> is <code>Completed</code> or <code>Terminated</code> using colour.</p>
  
  <pre><code>(ggplot(data=allData, aes(x=AnnounceDate, y=MarketProbability, colour=DealStatus))
   + geom_point() + scale_colour_hue(h = c(180,0)))
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/gaHQD.png" alt="enter image description here"></p>
  
  <p>I would also like to overlay a rolling Brier score, I have tried</p>
  
  <pre><code>(ggplot(data=allData, aes(x=AnnounceDate, y=MarketProbability, colour=DealStatus))
   + geom_point() + scale_colour_hue(h=c(180,0))
   + geom_line(aes(x=AnnounceDate, y=BrierScore)))
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/hue8x.png" alt="enter image description here"></p>
  
  <p>I am confused why two coloured lines are plotted? How do I dissociate the line with the previous plotted data?</p>