https://stackoverflow.com/questions/21802253/how-to-plot-a-heat-map-on-a-spatial-map
https://stackoverflow.com/questions/28059182/smoothing-out-ggplot2-map
https://stackoverflow.com/questions/43897148/colour-the-background-of-a-ggplot-polygon-to-match-the-lower-value-of-the-contou