title: One ggplot with multiple data sets
body: <p>I try to place points on a map so I am using <code>ggmap</code>. For the points, I have two separate data sets and the sample is below.<br>
  <br></p>
  
  <pre><code>dta1 = data.frame(storename = c(letters[1:5]),
                        storesize = c("small","small","medium","large","large"),
                        lon = c(10,11.2,15,19,22),
                        lat = c(5,5.8,5.6,6.1,3.4))
      dta2 = data.frame(population = sample(100,100,replace = T),
                        lon = runif(100,10,22),
                        lat = runif(100,3.5,6))
  </code></pre>
  
  <p><br>
  Here is the sample code for the plot. In my real data set, I have <code>ggmap(map)</code> instead of <code>ggplot()</code>. Why below code does not work?</p>
  
  <pre><code>ggplot() + geom_point(data=dta1,
                        aes(x=lon, y=lat,size = storesize), shape = 23,fill="blue") + 
    scale_size_manual(values = c(1,2,3)) +
    geom_point(data=dta2,
               aes(x=lon, y=lat,size = population), shape = 21,fill="orange")
  </code></pre>
  
  <p>If I run those separately like this, it works</p>
  
  <pre><code>ggplot() + geom_point(data=dta1,
                        aes(x=lon, y=lat,size = storesize), shape = 23,fill="blue") + 
    scale_size_manual(values = c(1,2,3))
  
  ggplot() +  geom_point(data=dta2,
               aes(x=lon, y=lat,size = population), shape = 21,fill="orange")
  </code></pre>
  
  <p>I would like to have one plot with two separate legends as each separated plot shows. <br> </p>
  
  <p>In addition, if I need to point one more particular place with a different shape (let's say <code>shape = 11</code>), how should I do it?</p>