title: Coloring the points in a plot based on another numeric variable
body: <p>I'm plotting "wt" versus "mpg" (from the "mtcars" dataset) and now I want to colour the points based on "hp". 
  I want to take the colour Green and to scale its shades, so the high "hp" values would get Dark Green and the low values would get Pale Green.</p>
  
  <p>base plot without colour:</p>
  
  <pre><code>data(mtcars)
  plot &lt;- with(mtcars, {
    plot(wt, mpg, xlab="Weight", ylab="Miles per Gallon")
  })
  </code></pre>
  
  <p>What is the simplest way to do this in R-Base? And what about ggplot2 / lattice?</p>