title: Omitting a Missing x-axis Value in ggplot2 (Convert range to categorical variable)
body: <p>I am using ggplot to generate a chart that summarises a race made up from several laps. There are 24 participants in the race,numbered 1-12, 14-25; I am plotting out a summary measure for each participant using ggplot, but ggplot assumes I want the number range 1-25, rather than categories 1-12, 14-25.</p>
  
  <p>What's the fix for this? Here's the code I am using (the data is sourced from a Google spreadsheet).</p>
  
  <pre><code>sskey='0AmbQbL4Lrd61dHlibmxYa2JyT05Na2pGVUxLWVJYRWc'
  library("ggplot2")
  require(RCurl)
  
  gsqAPI = function(key,query,gid){ return( read.csv( paste( sep="", 'http://spreadsheets.google.com/tq?', 'tqx=out:csv', '&amp;tq=', curlEscape(query), '&amp;key=', key, '&amp;gid=', curlEscape(gid) ) ) ) }
  
  
  sin2011racestatsX=gsqAPI(sskey,'select A,B,G',gid='13')
  sin2011proximity=gsqAPI(sskey,'select A,B,C',gid='12')
  
  h=sin2011proximity
  k=sin2011racestatsX
  l=subset(h,lap==1)
  
  ggplot() + 
  geom_step(aes(x=h$car, y=h$pos, group=h$car)) + 
  scale_x_discrete(limits =c('VET','WEB','HAM','BUT','ALO','MAS','SCH','ROS','SEN','PET','BAR','MAL','','SUT','RES','KOB','PER','BUE','ALG','KOV','TRU','RIC','LIU','GLO','AMB'))+ 
  xlab(NULL) + opts(title="F1 2011 Korea \nRace Summary Chart", axis.text.x=theme_text(angle=-90, hjust=0)) + 
  geom_point(aes(x=l$car, y=l$pos, pch=3, size=2)) + 
  geom_point(aes(x=k$driverNum, y=k$classification,size=2), label='Final') + 
  geom_point(aes(x=k$driverNum, y=k$grid, col='red')) + 
  ylab("Position")+ 
  scale_y_discrete(breaks=1:24,limits=1:24)+ opts(legend.position = "none") 
  </code></pre>