title: Changing size of points in R
body: <p>I would like to make a scatter plot using R base graphics, though I can use ggplot if it is necesary.  I have a data frame containing the x and y coordinates, and also two other covariates, call them a and b.  I would like the size of each point to vary with a (high values of a -> larger points) and the brightness/solidity of each point to vary with b (so that those points with low enough b are practically invisiable).  Does anyone have any advice on how to do this?  The documentation for R graphics seems comprehensive, but so general and high-level that I scarcely know where to begin.
  Thanks for your help</p>