r - Understand scale_gradient scaling

Question

stat_density2d(aes(fill = ..level..), alpha = .5, h = .02, n = 300, 
   geom = "polygon", data = accident_before) + 
scale_fill_gradient(low = "green", high = "red", guide = "colorbar", 
  name = "Level", breaks=seq(-40,80,30), limits=c(-40,80)) 

This is the code I am using but the output of this is a map that plots the city in grey color. What do low and high specify here? How should I understand which dataset is considered to output the map in what color.

question from:https://stackoverflow.com/questions/65840202/understand-scale-gradient-scaling

Answer 1

you are setting your scale limits to be -40 to 80, thus those will be the high and low colors. If you have grey values, this is either because you have NA values, or you have values out of this range.

Check ?ggplot2::scale_fill_gradient for a bit more information.

The underlying gradient is calculated with scales::seq_gradient_pal()