title: R heat map: Ordering by value; label issues
body: <p>I am looking to improve upon output I implemented in R based on Jeromy's answer <a href="https://stackoverflow.com/questions/5453336/plot-correlation-matrix-into-a-graph">here</a> (thanks!).  Mine is a 31x31 matrix with positive and negative values, and uses basically the same ggplot2 code:  </p>
  
  <pre><code>library(ggplot2)
  library(reshape)
  
  z&lt;-cor(insheet3,use="complete.obs",method="kendall")
  zm&lt;-melt(z)
  ggplot(zm, aes(X1,X2, fill=value)) +  geom_tile() +
  scale_fill_gradient2(low = "blue",  high = "dark violet")  
  </code></pre>
  
  <p>I need to change three things:  </p>
  
  <ol>
  <li><p>Right now, the rows appear in reverse alphabetical order, which means no visible data trends. <strong>How can I influence the order of the rows and columns</strong>, such that either:</p>
  
  <p>A. (Preferred:) The columns are <em>ordered by correlation value</em> (negative to positive or vice versa), as they are in the ellipse package output on that same page;  or  </p>
  
  <p>B. The columns are <em>manually ordered</em>, so that I can group similar variables?  </p></li>
  <li><p>Along the bottom X-axis, my variable names are overlapping dramatically and are unreadable.  They need to remain long (i.e., OrthoPhos, Ammonia, Residential...), so <strong>how can I rotate their labels 90 degrees</strong>?</p></li>
  <li><p>Is there a way to <strong>remove the "X1" and "X2" labels</strong> along each axis?  </p></li>
  </ol>
  
  <p>Thank you!</p>