Invert colored area geom_area

Question

I am sure it is easy to solve this question.

I am trying to fill the area below the line I plot from "mydata" using ggplot2.

here the example:

a = seq(10, 100, 5)
b = -(runif(19, 0, 50))
mydata <- data.frame(a, b)

ggplot(data = mydata, aes(x = a, y = b)) +
  geom_area()

geom_bar fills above the line, considering the area the one from 0 and the values (negative). I want to fill the other area and I think I should delimit the area I want to colour, but I do not know how.

The script above is just an easy example. My real script is

ggplot(data = ctd, aes(x = Longitude, y = Depth)) +
  geom_raster(aes(fill = NO3_uM)) +
  scale_fill_gradientn(colours = rev(my_colours)) +
  geom_contour(aes(z = NO3_uM), binwidth = 2, colour = "black", alpha = 0.2) +
  #geom_contour(aes(z = NO3_uM), breaks = 20, colour = "black") +

  geom_point(data = ctd, aes(x = Longitude, y = Depth),
             colour = 'black', size = 3, alpha = 1, shape = 15) +
  geom_area(data = trsect, aes(x = Longitude, y = Depth), fill = "black")+
  ylim(-320,0)

my result is:

I want to colour the other area of the "mountain"

Answer 1

You can use geom_ribbon for this. geom_area is a special case of geom_ribbon where ymin is set to 0. Instead, set ymin to the minimum of your y-values, i.e. ymin = min(b). Or if you wanted some extra space, you could do ymin = min(b) - 2 or something like that.

library(tidyverse)

a = seq(10, 100, 5)
b = -(runif(19, 0, 50))
mydata <- data.frame(a, b)

ggplot(mydata) +
    geom_ribbon(aes(x = a, ymin = min(b), ymax = b))

Created on 2018-05-04 by the reprex package (v0.2.0).

Answer 2

You can try to make a vector to define a new area.

qq = rep(-60, length(b))
ggplot(data=mydata, aes(x = a, y = b))+
 geom_area(data=data.frame(qq), aes(y=qq), fill='red', alpha=0.5) +
 geom_area(aes(y=b), col='black') 

Which produces

You can play with fill and the theme to have a different set of options (It is not quite clear what's your desired output).

ggplot(data=mydata, aes(x = a, y = b))+
   geom_area(data=data.frame(qq), aes(y=qq),fill='red', alpha=0.5) +
   geom_area(aes(y=b), fill='white') + theme_void()

Which produces