title: Order data inside a geom_tile
body: <p>i have a data frame and i want to generate a <code>geom_tile()</code> plot from it , but I want the graph to be ordered not based on the alphabetic but based on a variable inside this data frame.</p>
  
  <pre><code>structure(list(V1 = c("a", "y", "w", "p", "v", "h", "i"), 
      V2 = c("r", "w", "q", "m", "l", "q", "g"), V3 = c( 
      "5", "2", "9", "2", "1", "3", "0")), .Names = c("V1", "V2", 
  "V3"), class = "data.frame", row.names = c(NA, -8L))
  </code></pre>
  
  <p>I want to order the plot based in the variable <code>V3</code> , because the normal plotting will order them based on the alphabetic in <code>V1</code> and <code>V2</code> .</p>
  
  <p>How this can be done ??</p>