R - adding legend to ggmap (ggplot2) while using annotate

Question

FYI: I\'m fairly new to ggplot2 and ggmap so I apologize for the sloppy code but it is the only way I\'ve been able to plot sets of groups of points where each group has it\'s o

Answer 1

Instead of using annotate, here's a method that adds a point layer using geom_point. Almost any geom can be added to a ggmap object as it would be added to a ggplot object. Because Size (see the contents of the df data frame below) is a colour aesthetic in the call to geom_point, the legend is generated automatically.

library(ggmap)

# Get a map - A map of Canberra will do
ACTmap = get_map(c(149.1, -35.325), zoom = 12, source = "google", maptype = "roadmap")

# A data frame of lon and lat coordinates and the variable Size
df = data.frame(lon = c(149.0307, 149.1326, 149.089, 149.048, 149.0965),
            lat = c(-35.3892, -35.28225, -35.34005, -35.34857, -35.34833),
            Size = c(1,2,3,4,5))

# Draw the map
ACTmap = ggmap(ACTmap)

# Add the points
ACTmap = ACTmap + geom_point(data = df, aes(x = lon, y = lat, colour = Size), 
      alpha = 0.6,  size = 10)

# Change the legend
ACTmap + scale_colour_continuous(low = "red", high = "blue", space = "Lab", guide = "colorbar")