For the first time, let's keep it simple and load some package-bundled polygons as the base map. To this end, we will use the maps package. After loading it, we use the map function to render the polygons of the United States of America, add a title, and then some points for the airports and also for Houston with a slightly modified symbol:

> library(maps)
> map('state')
> title('Flight destinations from Houston,TX')
> points(h$lon, h$lat, col = 'blue', pch = 13)
> points(dt$lon, dt$lat, col = 'red', pch = 19)

And showing the airport names on the plot is pretty easy as well: we can use the well-known functions from the base graphics package. Let's pass the three character names as labels to the text function with a slightly increased y value to shift the preceding text the previously rendered data points:

> text(dt$lon, dt$lat + 1, labels = dt$Dest, cex = 0.7)

Now, we can also specify the color of the points to be rendered. This feature can...