Leaflet legend in R based on color and shape

Question

I have icons on a leaflet plot which have different colors and shapes based on some variables in my data frame. I want to include a legend on the plot that shows what each shape and color combination represents. How should I do it?

To demonstrate with dummy data:

 library(leaflet)

 lat1= 36+runif(n=5,min=-1,max=1)

 lon1 =-115+runif(n=5,min=-1,max=1)

lat2= 35+runif(n=5,min=-0.5,max=0.5)
lon2 =-110+runif(n=5,min=-0.5,max=0.5)

lat3= 34+runif(n=5,min=-0.5,max=0.5)
lon3 =-112+runif(n=5,min=-0.5,max=0.5)
data_all=rbind(data.frame(Longitude=lon1,Latitude=lat1,Group=sample(c(16,17,18),5,replace = TRUE),condition=sample(c("red","blue","green"),5,replace = TRUE),stringsAsFactors = FALSE),
           data.frame(Longitude=lon2,Latitude=lat1,Group=sample(c(16,17,18),5,replace = TRUE),condition=sample(c("red","blue","green"),5,replace = TRUE),stringsAsFactors = FALSE),
           data.frame(Longitude=lon3,Latitude=lat1,Group=sample(c(16,17,18),5,replace = TRUE),condition=sample(c("red","blue","green"),5,replace = TRUE),stringsAsFactors = FALSE))

# A function to create png images for each shape and color 
pchIcons = function(pch = 1, width = 30, height = 30, bg = "transparent", col = NULL, ...) {
  n = length(pch)
  files = character(n)
  # create a sequence of png images
  for (i in seq_len(n)) {
    f = tempfile(fileext = '.png')
    png(f, width = width, height = height, bg = bg)
    par(mar = c(0, 0, 0, 0))
    plot.new()
    points(.5, .5, pch = pch[i], col = col[i], cex = min(width, height) / 8, ...)
    dev.off()
    files[i] = f
  }
  files
}
### ---------


leaflet(data_all)%>% addTiles() %>%
  addMarkers(
    data = data_all,
    icon = ~ icons(
      iconUrl = pchIcons(pch= Group,width=40,height=40,col=condition,lwd=4),
      popupAnchorX = 20, popupAnchorY = 0
    ))

Answer 1

Based on this post, using base64enc and creating fixed filenames instead of using tempfile:

# A function to create file names 
filename <- function(pch,col) paste0(pch, '_', col, '.png')

# A function to create png images for each shape and color 
pchIcons = function(pch = 1, width = 30, height = 30, bg = "transparent", col = NULL, ...) {
  n = length(pch)
  files = character(n)
  # create a sequence of png images
  for (i in seq_len(n)) {
    f = filename(pch[i],  col[i])
    png(f, width = width, height = height, bg = bg)
    par(mar = c(0, 0, 0, 0))
    plot.new()
    points(.5, .5, pch = pch[i], col = col[i], cex = min(width, height) / 8, ...)
    dev.off()
    files[i] = f
  }
  files
}

# A function to build the legend
build_legend <- function(){
  paste(sapply(strsplit(unique(paste(data_all$Group,data_all$condition)), " "), 
               function(x){
                 paste0("<img src='data:image/png;base64,",
                        base64enc::base64encode(filename(x[[1]], x[[2]])), 
                        "' width='16'; height='16'> ",
                        "Group=",x[[1]], " Condition=", x[[2]], 
                        "<br/>" )}), collapse = " ")
}

# The plot
leaflet(data_all)%>% addTiles() %>%
  addMarkers(
    data = data_all,
    icon = ~ icons(
      iconUrl = pchIcons(pch= Group,width=40,height=40,col=condition,lwd=4),
      popupAnchorX = 20, popupAnchorY = 0)) %>% 
  addControl(html = build_legend(), position = "bottomleft")