title: R: wireframe like 3D plot for categorical variables
body: <p>I want to develop wireframe like plot with non-numeric on X, Y axis however numeric in Z axis. </p>
  
  <pre><code># mydata 
  set.seed(123)
  yv &lt;- rnorm(20, 10, 3)
  gen &lt;- rep(paste("G", 1:5, sep= ""), 4)
  env &lt;- c(rep(c("CA","MN","SD", "WI"), each = 5))
  mdf &lt;- data.frame(yv, gen, env) 
  </code></pre>
  
  <p>I tried using lattice:    </p>
  
  <pre><code>require(lattice)
  wireframe(yv,gen, env, data = mdf)
  
  Error in UseMethod("wireframe") : 
    no applicable method for 'wireframe' applied to 
    an object of class "c('double', 'numeric')"
  </code></pre>
  
  <p>Any suggestions appreciated. </p>