r - How do I adjust visibility of gridlines in ggplot chart

Question

The code below creates a scatter plot and uses theme_bw with has gridlines in the background -

data = mtcars

data %>% 
  select(mpg, disp) %>% 
  ggplot(aes(disp, mpg))+
  geom_point(size = 3)+
  theme_bw()

I would like to also include some vertical and horizontal lines on the chart. However with the current gridlines, it looks a bit busy. Is there a way to further reduce the visibility of the gridlines. I don't want want remove them completely.

question from:https://stackoverflow.com/questions/65908472/how-do-i-adjust-visibility-of-gridlines-in-ggplot-chart

Answer 1

This could be achieved by switching to a lighter color or by reducing the opacity of the color used for the grid lines which both could be achieved via theme option panel.grid. Below I show the second approach. Unfortunately element_line has no alpha argument to set the opacity but you could adjust it via the hex color code:

1. I make use of the default grid line color "grey92" which has rgb values (235, 235, 235).

2. To set the opacity I use rgb() which as a fourth argument takes the opacity or alpha which I reduce to a value of 100:

library(ggplot2)
library(dplyr)

data = mtcars

# Reduce the opacity of the grid lines: Default is 255
col_grid <- rgb(235, 235, 235, 100, maxColorValue = 255)

data %>% 
  select(mpg, disp) %>% 
  ggplot(aes(disp, mpg))+
  geom_point(size = 3)+
  theme_bw() +
  theme(panel.grid = element_line(color = col_grid))