Suppose we want to plot this data:

library(ggplot2)
library(sf)
library(raster)
library(colorRamps)

min_lon <- 10
max_lon <- 17
min_lat <- 8
max_lat <- 17
grid_size <- 0.5
lon_grids <- 1   ((max_lon - min_lon)/grid_size)
lat_grids <- 1   ((max_lat - min_lat)/grid_size)
points <- data.frame(lon = rep(seq(min_lon, max_lon, grid_size), lat_grids), lat = rep(seq(min_lat, max_lat, grid_size), each = lon_grids))

points$Var <- runif(min= 10, max = 48,  285)
points$value <-cut(points$Var, breaks= seq(10.08, 47.80, length.out = 13), dig.lab = 1)
ggplot()  
  coord_sf(xlim = c(min_lon, max_lon), ylim = c(min_lat, max_lat))  
  theme_bw() 
  geom_raster(data = points, aes(x = lon, y = lat, fill = value), interpolate = FALSE)  
  labs(x="Longitude", y="Latitude") 
  scale_fill_manual(values = matlab.like(n = 13), name = "[m]", 
                    labels = sprintf("%.2f", seq(10.08, 47.80, length.out = 13)),
                    guide = guide_legend(reverse = TRUE)) theme(legend.position = "bottom")

This code produces the following graph:

Two problems I am facing here:

1. To make it discrete, I used the cut function. I chose the breaks= seq(10.08, 47.80, length.out = 13) arbitrary based on the minimum and maximum values with a random length of 13. Is there any criteria to decide the correct range?

2. Is there any way to make the legend look like this?

CodePudding user response：

One option would be to use e.g. scale_fill_stepsn with guide_binswhich does not require to manually discretize the variable mapped on fill. Additionally I use a custom function to set the breaks of the legend instead of the default mechanism to set the number of breaks.

set.seed(123)

library(ggplot2)
library(colorRamps)

base <- ggplot()  
  coord_sf(xlim = c(min_lon, max_lon), ylim = c(min_lat, max_lat))  
  theme_bw()  
  geom_raster(data = points, aes(x = lon, y = lat), interpolate = FALSE)  
  labs(x = "Longitude", y = "Latitude")  
  theme(legend.position = "bottom")

base  
  aes(fill = Var)  
  scale_fill_stepsn(colors = matlab.like(n = 13), name = "[m]",
                    breaks = function(x) seq(x[[1]], x[[2]], length.out = 13),
                    labels = ~ sprintf("%.0f", .x),
                    guide = guide_bins(axis = FALSE,
                                       show.limits = TRUE))