title: How to set ggplot alpha (transparency) value for all points at once
body: <pre><code>library(data.table)
  library(ggpolot2)
  
  numPoints &lt;- 10000
  dt &lt;- data.table(a=rnorm(numPoints),b=rnorm(numPoints))
  
  qplot(a,b,data=dt, geom="point", alpha=1)
  qplot(a,b,data=dt, geom="point", alpha=0.1)
  qplot(a,b,data=dt, geom="point", alpha=0.01)
  </code></pre>
  
  <p>Regardless of the alpha value I choose, the resulting chart seems to have the same amount of transparency. </p>
  
  <p>How can I get the points to be more transparent (so that the density of the points in an area is more clearly visible)?</p>