Change xaxis label to specific letters R
<p>Im making a lot of plots in R and a part of my code looks like this: </p>

<pre><code>plot(x=0,y=0, type="n", ylim=c(0,250), xlim=c(0,8), bty="n", main = "Line 20 male 3 sec rep 2", 
     xlab = "Concentration", ylab = "MM above buttom")
fc &lt;- levels(dat20m2$Conc)
for(i in 1:length(fc)){
  tmp &lt;- dat20m2[dat20m2$Conc==fc[i],]
  points(y=tmp$t30.sum,x=rep(i,length(tmp$t30)))
  points(y=mean(tmp$t30.sum),x=i,col="red")
}
abline(lm(t30.sum~as.integer(Conc),data=dat20m2), lty=2)
</code></pre>

<p>My x axis is from 0:8 but what i would like it to be is A, B, C, D, E, X, Y and i can make it work with the alphabet but when i wanna jump some letters i get into troubles. Can someone help me with that? </p>