How to create a horizontal gradient under a curve?

Question

I'm attempting to recreate the following plot styling in R:

The data doesn't need to map onto the gradient in any meaningful way (though I guess technically it somewhat maps on to the categorical steps of the x-axis, with the colour gradually changing as the date/month changes).

I've tried scale_colour_gradient() and scale_fill_gradient(), but I'm not having any luck. Any guidance on how I could achieve something similar would be appreciated!

Sample data:

    views <- tribble(
      ~month, ~views,
      "Jan", 374,
      "Feb", 500,
      "Mar", 416,
      "Apr", 603,
      "May", 389,
      "Jun", 510) %>%
      transform(month = factor(month, levels=c("Jan","Feb","Mar","Apr","May","Jun")))

    views %>%
      ggplot(., aes(x = month, y = views, group=1)) +
      geom_line(color="red") +
      scale_y_continuous(breaks=seq(0, 1000, by = 200),
                             limits = c(0, 1000))

Two links that may help https://www.r-bloggers.com/2021/07/little-useless-useful-r-functions-colourful-ggplot-line-graphs/ — Yacine Hajji, Nov 17 '22 at 16:56
https://stackoverflow.com/questions/37241893/is-it-possible-to-apply-color-gradient-to-geom-smooth-with-ggplot-in-r — Yacine Hajji, Nov 17 '22 at 16:57
related https://stackoverflow.com/questions/53397131/gradient-fill-in-ggplot2 and https://stackoverflow.com/questions/27250542/how-to-make-gradient-color-filled-timeseries-plot-in-r?noredirect=1&lq=1 and https://stackoverflow.com/questions/64691000/create-top-to-bottom-fade-gradient-geom-density-in-ggplot2 — tjebo, Nov 17 '22 at 17:27

tjebo · Answer 1 · 2022-11-17T17:29:28.703

You can make a horizontal gradient by creating segments along your "polygon" - its lower boundaries are the x axis, the upper boundaries your connecting line. The trick is to find the coordinates for the connecting lines - I am approximating (!) them with approx.

For the color gradient in geom_line, you can use ggforce::geom_link2 - conveniently, you can use the same color scale for both segments and geom_link2.

library(tidyverse)
views <- tribble(
  ~month, ~views,
  "Jan", 374,
  "Feb", 500,
  "Mar", 416,
  "Apr", 603,
  "May", 389,
  "Jun", 510) %>%
  transform(month = factor(month, levels=c("Jan","Feb","Mar","Apr","May","Jun")))

foo <- data.frame(approx(1:6, views$views, n = 2000))

ggplot(views, aes(x = month, y = views, group=1)) +
  ggforce::geom_link2(aes(color = as.integer(month)), linewidth = 1) +
  geom_segment(data = foo, aes(x, xend = x, y = 0, yend = y, color = x), alpha = .5, linewidth = .1) +
  scale_color_gradientn(colours = c("#83DFEF", "#6F9EF2", "#815BF6")) +
  theme(legend.position = "none")

^{Created on 2022-11-17 with reprex v2.0.2}

score 1 · Answer 2 · answered Nov 17 '22 at 17:30

First credit to @tjebo for a great approach with segments. Another option using geom_line by converting the months first to digits. Later you can convert the digits back to months on axis like this:

# Data from tjebo 
foo <- data.frame(approx(1:6, views$views, n = 2000))

library(ggplot2)
library(dplyr)
views %>%
  mutate(month_dig = as.numeric(unclass(month))) %>%
  ggplot(.) +
  geom_segment(data = foo, aes(x, xend = x, y = 0, yend = y, color = x), alpha = .5, size = .1) +
  scale_colour_gradient(low = 'lightblue', high = 'purple') +
  geom_line(aes(x = month_dig, y = views, group=1, colour = month_dig)) +
  scale_x_continuous(breaks = seq(1,6,1), labels = month.abb[1:6]) +
  scale_y_continuous(breaks=seq(0, 1000, by = 200), limits = c(0, 1000)) +
  theme_bw() +
  labs(x = 'Month', y = 'Views', fill = 'Gradient')
#> Warning: Using `size` aesthetic for lines was deprecated in ggplot2 3.4.0.
#> ℹ Please use `linewidth` instead.

^{Created on 2022-11-17 with reprex v2.0.2}

How to create a horizontal gradient under a curve?

2 Answers2