title: Changing the alpha values in R{graphics} while the colour argument is used
body: <p>I'm so used to doing this in ggplot2 that I'm having a very hard time figuring out how to specify alpha values using R base graphics, while the col= argument in plot() is used for assigning a colour type to a categorical variable. </p>
  
  <p>Using the iris data set (although in this context it doesn't really make sense why we would need to change the alpha values)</p>
  
  <pre><code>data(iris)
  library(ggplot2)
  g &lt;- ggplot(iris, aes(Sepal.Length, Petal.Length)) + geom_point(aes(colour=Species), alpha=0.5) #desired plot
  
  plot(iris$Sepal.Length, iris$Petal.Length, col=iris$Species) #attempt in base graphics
  </code></pre>
  
  <p>What about mapping another variable to the alpha value using {graphics}?
  For example in ggplot2:</p>
  
  <pre><code>g2 &lt;- ggplot(iris, aes(Sepal.Length, Petal.Length)) + geom_point(aes(colour=Species, alpha=Petal.Width)) 
  </code></pre>
  
  <p>Any help is appreciated!</p>