title: ggplot scatterplot and lines
body: <p>I have some biological data for two individuals, and I graph it using R as a scatterplot using ggplot like this:</p>
  
  <pre><code>p1&lt;-ggplot(data, aes(meth_matrix$sample1, meth_matrix$sample3)) +
    geom_point() +
    theme_minimal()
  </code></pre>
  
  <p>which works perfect, but I want to add lines to it: the abline that divides the scatterplot in half:</p>
  
  <pre><code>p1  + geom_abline(color="blue")
  </code></pre>
  
  <p>and my question is: how can I draw two red lines parallel to that diagonal (y intercept would be 0.2, slope would be the same as the blue line) ??</p>
  
  <p>Also: how can I draw the difference of both samples in a similar scatterplot (it will look like a horizontal scatterplot) with ggplot? right now I can only do it with plot like:</p>
  
  <pre><code>dif_samples&lt;-meth_matrix$sample1- meth_matrix$sample3
  plot(dif_samples, main="difference", 
       xlab="CpGs ", ylab="Methylation ", pch=19)
  </code></pre>
  
  <p>(also I'd like adding the horizontal blue line and the red lines paralllel to the blue line)</p>
  
  <p>Please help!!!</p>
  
  <p>Thank you very much.</p>