title: How to make a designated gene point to be on the top layer of the volcanoplot
body: <p>I'm trying to make the volcanoplot using my DEGs data with ggplot2, I have 2 groups genes of interest in the DEGs, and I want to display these 2 groups of genes with colors and appear on the top layer of the final volcanoplot. The structure of my dataset as follows:</p>
  
  <pre><code>Genes  Log2FoldChange   pvalue  Groups
  A      -1.22            0.003   a
  B      -2.23            0.001   a
  C      2.52             0.004   b
  D      3.23             0.008   b
  E      3.43             0.005   c
  F      -2.45            0.001   c
  H      -0.56            0.432   c
  ...
  </code></pre>
  
  <p>Group "a" and "b" include about 20 genes, Group "c" include other thousands of genes. </p>
  
  <p>I used the ggplot2 code to make a volcanoplot, but most of the gene plot of Group "a" and "b" (red and blue) were covered by the plots of Group "c" (grey), how can I force all the gene plots of Group "a" and "b" on the top layer of the Group "c" in order to they can be all displayed. </p>
  
  <pre><code>r03=ggplot(data,aes(log2FoldChange, -log10(pvalue)))
  
  r03+geom_point(aes(color=Groups))
  +scale_color_manual(values=c("blue","grey","red"))
  +geom_hline(yintercept=1.3, linetype=3)
  +geom_vline(xintercept=c(-1,1), linetype=3)
  </code></pre>