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 be used to plot our first meaningful map to highlight the number of flights in 2011 to different parts of the USA:

> map('state')
> title('Frequent flight destinations from Houston,TX')
> points(h$lon, h$lat, col = 'blue', pch = 13)
> points(dt$lon, dt$lat, pch = 19,
+   col = rgb(1, 0, 0, dt$N / max(dt$N)))
> legend('bottomright', legend = round(quantile(dt$N)), pch = 19, 
+   col = rgb(1, 0, 0, quantile(dt$N) / max(dt$N)), box.col = NA)

So the intensity of red shows the number of flights to the given points (airports); the values range from 1 to almost 10,000. Probably it would be more meaningful to compute these values on a state level as there are many airports, very close to each other, which might be better aggregated at a higher administrative area level. To this end, we load the polygon of the states, match the points of interest (airports) with the overlaying polygons (states), and render the polygons as a thematic map instead of points, like we did on the previous pages.