How to make continuous legend in ggplot2?

Question

I have little query regarding the legend in ggplot2. How can I make legend in a continuous level from the attached code. In this code the legend is in discrete form I want to make it in continuous form (smooth continuous pattern) The code is reproducible ( little bit long but probably one line will change in this code)

 library(sf)
 library(sp)

 r <- raster(t((volcano[,ncol(volcano):1] - 94) * 4.95))

 # Let's mock with a shapefile
 poly <- st_as_sfc(st_bbox(st_as_sf(rasterToPolygons(r))))

 # Sample 4 points
 set.seed(3456)

 sample <- st_sample(poly, 4)
 sample <- st_buffer(sample, c(0.01, 0.02, 0.03))
 sample <- st_sf(x=1:4, sample)
 st_write(sample, "1aa.shp", append = FALSE)
 # Mocked data

 # Now let's start with code -------

 r <- raster(t((volcano[,ncol(volcano):1] - 94) * 4.95))

 # Use sf!!
 pg <- st_read("1aa.shp") # loadshapfile 
 plot(r)
 plot(st_geometry(pg), add= TRUE,) # it appears here like first picture 
(left).

 centile90 <- quantile(r, 0.90)
 df <- as.data.frame(as(r, "SpatialPixelsDataFrame"))
 colnames(df) <- c("value", "x", "y")

 library(ggplot2)

  mybreaks <- seq(0, 500, 50)

  ggplot(df, aes(x, y, z = value)) +
  geom_contour_filled(breaks = mybreaks) +
  geom_contour(breaks = centile90, colour = "pink",
             size = 0.5) +
   # And here we have it
   geom_sf(data=pg, fill="black", inherit.aes = FALSE) +
  scale_fill_manual(values = hcl.colors(length(mybreaks)-1, "Zissou1", rev 
  = FALSE)) +
  scale_x_continuous(expand = c(0, 0)) +
  scale_y_continuous(expand = c(0, 0)) +
  theme_classic() +
  theme()

After run this code I got legend in this form (left side of attached picture) ,means discontinuous, but I want to make this legend look like (right side of attached picture). But in my code I give the command of continuous scale. Can someone tell me how to make this code for required legend smooth continuous pattern.

To confirm, the chart itself is showing a small number of discrete color bins but you want the legend to show a smooth spectrum? — Jon Spring, Aug 14 '22 at 08:37
At least try to create a [mwe](http://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example), now we are just guessing and any example we create, might not work in your case as you might be using different packages etc etc etc. — phiver, Aug 14 '22 at 08:41
Does the example in the help for `geom_contour_filled` demonstrate the issue you're asking about? `ggplot(faithfuld, aes(waiting, eruptions, z = density)) + geom_contour_filled()` — Jon Spring, Aug 14 '22 at 09:05
Yes I want a legend which shows a smooth spectrum. And this example executes clearly if you have a one tiff file and one shape file you can easily run this example for **geom_contour_filled**. _John Spring — user_3264, Aug 14 '22 at 10:54
Can you tell me in which line I need to add this **ggplot(faithfuld, aes(waiting, eruptions, z = density)) + geom_contour_filled()**. Because I am very bad right now in this coding skill _John Spring — user_3264, Aug 14 '22 at 10:55
I am sorry for poor convivence because I am learning the skill so might be my question creates some confusion but I will try to overcome these Problem -phiver — user_3264, Aug 14 '22 at 10:59

score 8 · Accepted Answer · answered Aug 16 '22 at 11:08

You could use the function geom_tile and scale_fill_gradientn to get a continuous scale instead of discrete scale legend like this:

library(ggplot2)
library(sf)
library(sp)
library(raster)
mybreaks <- seq(0, 500, 50)

ggplot(df, aes(x, y, fill = value)) +
  geom_tile() +
  scale_fill_gradientn(
    colours = hcl.colors(length(mybreaks)-1, "Zissou1", rev = FALSE), 
    breaks = mybreaks
  ) +
  geom_sf(data=pg, fill="black", inherit.aes = FALSE) +
  scale_x_continuous(expand = c(0, 0)) +
  scale_y_continuous(expand = c(0, 0)) +
  theme_classic() +
  theme()

Output:

How to make continuous legend in ggplot2?

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