title: adding data to existing ggplot2 plot
body: <p>I am using the ggplot2 package on the cars dataset. I have created a grid based on the manufacturer and color coded by class..</p>
  
  <p>What I would like to do is:
  add to each plot in the grid from the dataset mdat.Data should be represented as data points.
  The output I would like to see is the data in mdat is to be represented in each plot as dots, the class should be added to the legend, I would like each point to be represented with a different color and shape. I am not sure how this can be done and any help is appreciated. </p>
  
  <p>Thanks so much!</p>
  
  <pre><code>## Sample data
     p &lt;- ggplot(mpg, aes(x=hwy, y=cty))
     p&lt;- p + facet_grid(. ~ manufacturer) + facet_wrap(~manufacturer) 
     p&lt;- p + geom_point(aes(colour = class), size = 7)
     p&lt;- p + scale_colour_brewer()
     p&lt;- p + geom_point(shape = 1, size = 7,  alpha = I(0.7))
     print(p) 
  
  
  
   ##now add on the randomn portfolios on another grid
     hwy = c("35", "40", "25")
     cty = c("20", "25", "10")
     mdat &lt;- data.frame(hwy,cty)
     mdat$class &lt;- c("generic1", "generic2", "generic3")
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/oTmgP.jpg" alt="enter image description here"></p>