title: Implement data from several data frames in one bubble plot
body: <p>I have a data frame with the dimensions 625616 x 12. I would like to illustrate the data with a bubble plot. To illustrate my situation I will use the mtcars data set. </p>
  
  <pre><code>mtcars$cyl = as.factor(mtcars$cyl)
  bp = ggplot(as.data.frame(mtcars), aes(x = wt, y = mpg, size = qsec)) + geom_point(shape = 21)
  bp
  </code></pre>
  
  <p>Analogous to my data frame, I used with this command the data from 3 out of 12 columns. Ideally, I would like to add to this bubble plot another set of bubbles in another colour (column 4-6).</p>
  
  <p>I tried to use the "add" function. </p>
  
  <pre><code>bp2 = ggplot(as.data.frame(mtcars), aes(x = wt2, y = mpg2, size = qsec2)) + geom_point(shape = 21)
  plot(bp2, add = T)
  </code></pre>
  
  <p>Unfortunately it doesn't work out neither. </p>