Putting it all together

You now have a nice map of Corvallis, but how do you put the locations of the house sales on top?

Similar to ggplot(), you can add layers of data to a ggmap() call (e.g. + geom_point()). It's important to note, however, that ggmap() sets the map as the default dataset and also sets the default aesthetic mappings.

This means that if you want to add a layer from something other than the map (e.g. sales), you need to explicitly specify both the mapping and data arguments to the geom.

What does this look like? You've seen how you might make a basic plot of the sales:

ggplot(sales, aes(lon, lat)) + 
  geom_point()

An equivalent way to specify the same plot is:

ggplot() + 
  geom_point(aes(lon, lat), data = sales)

Here, we've specified the data and mapping in the call to geom_point() rather than ggplot(). The benefit of specifying the plot this way is you can swap out ggplot() for a call to ggmap() and get a map in the background of the plot.