A frequent challenge of visualization is behind the scenes, to get the data and to mold it into the format you need. Do that. Then map.
Working with Line Maps, the Google Places API, and R
How to Make a Contour Map
Filled contour plots are useful for looking at density across two dimensions and are often used to visualize geographic data. It’s straightforward to make them in R — once you get your data in the right format, that is.