title: ggplot: how to move the 0 coordinate to the middle of X-axis?
body: <p><img src="https://i.stack.imgur.com/DlAIM.png" alt="enter image description here"></p>
  
  <p>I am trying to create a volcano plot using the following code, but I would like to put the "0" coordinate to the middle of the X-axis. Is there a way to do this in ggplot?</p>
  
  <pre><code>v&lt;-ggplot(exprData.fil,aes(Effect,Effect.sig))+geom_point(aes(colour=Effect.sig),alpha=0.7)+scale_colour_gradient(low="red",high="green")
      v+ggtitle(mainTitle)
      v+xlab(expression(log[2](bar(After) / bar(Before))))+ylab(expression(-log[10]("p.value"))) 
  </code></pre>