Annotate map with circles

circle_map_annotation

# Prep ----
library(ggplot2)
crimes <- data.frame(state = tolower(rownames(USArrests)), USArrests)
crimesm <- reshape2::melt(crimes, id = 1)
states_map <- map_data("state") 

# Make your map ------
  map <- ggplot(crimes, aes(map_id = state)) +
    geom_map(aes(fill = UrbanPop), map = states_map) +
    expand_limits(x = states_map$long, y = states_map$lat) +
    coord_map(projection = "polyconic") + 
    theme_void() +
    theme(legend.position = "none")
  map
          
# Make the map a grid graphical object (“grob”)  ------
# https://stat.ethz.ch/R-manual/R-devel/library/grid/html/grid.grob.html
  grobMap <- ggplotGrob(map)

# Make a container canvas  ------
  df <- data.frame(x = 1:10, y = 1:10)
  container_plot <- ggplot(df, aes(x, y)) +
    geom_blank() +
    theme_void() 
  
  container_plot
  
# Add a custom annotation. To do that, you pass it a grob ----
  container_plot +
    annotation_custom(grob = grobMap) 
  
# Then add your circle annotation / custom ribbon annotation ------
#' alternatively ggforce::geom_circle?
#' http://stackoverflow.com/a/39071128/4386615
  gg_circle <- function(r, xc, yc, color="black", fill=NA, ...) {
    x <- xc + r*cos(seq(0, pi, length.out=100))
    ymax <- yc + r*sin(seq(0, pi, length.out=100))
    ymin <- yc + r*sin(seq(0, -pi, length.out=100))
    annotate("ribbon", x=x, ymin=ymin, ymax=ymax, color=color, fill=fill, ...)
  }

# Also remove text and ticks and lines in theme ----
  container_plot +
    annotation_custom(grob = grobMap) + 
    gg_circle(r=1, xc=2.5, yc=6, fill="red", alpha=.5) +
    geom_text(aes(label="65%"), x=2.5, y=6) +
    theme_void()