R：彩色gps点显示的颜色更暗

Question

I would like to plot gps points like a heat map - those that show up more times will be darker/warmer in color. My data look like this:

       lon   lat Freq
1  -121.93 68.28    1
2  -117.72 70.72    1
3  -110.69 68.75    1
4   -94.79 61.03    1
5   -93.80 58.63    1
6   -92.18 62.88    1
7   -89.68 46.15    3
8   -83.00 62.50    1
9   -78.99 56.41    1
10  -78.87 56.39    1
11  -71.30 65.95    1
12  -67.36 62.84    1
13  -67.35 62.86    1
14  -67.25 63.45    2

I can plot these data as follows:

newmap<-getMap(resolution="low")
plot(newmap)
points(data3$V5, data3$V4, col = "red", pch = 15, cex = 0.5)

However, I would like the points that have a frequency of 2 or 3 (see Freq column) to be darker/warmer like a heat map, with an accompanying legend. It looks like there might be a solution using ggmap, but I have not been able to obtain a world map using ggmap. (I need a world map because I will be plotting far more gps points than in this example that cross the globe). I like the way this outline of the world looks.

Answer 1

One option would be to set the opacity using rgb instead of just putting in a color. For instance

points(data3$V5, data3$V4, col = rgb(1, 0, 0, 0.5), pch = 15, cex = 0.5)

Answer 2

With ggplot2:

library(ggmap)
library(ggplot2)
world <- map_data("world")
ggplot() + geom_path(data=world, aes(x=long, y=lat, group=group)) +
geom_point(data=data3, aes(x=lon, y=lat, colour=Freq, size=sqrt(Freq/pi))) +  scale_colour_gradientn(colours = rev(heat.colors(12))) +
  scale_size(guide = FALSE)