If you specify a vector of gridline colors, ggplot2 cycles through them

panel-gridlines-color-vector.R

library(tidyverse) # dplyr, stringr, ggplot2
library(lubridate)

#' The `sunspot.month` dataset as a tibble:
sunspots_reshaped <- 
  tibble(
    sunspot_month = sunspot.month %>% 
      start() %>% 
      c(1) %>% 
      paste(collapse = "-") %>% 
      ymd() + 
      months(1:length(sunspot.month) - 1),
    sunspots = sunspot.month
  )

#' A function for labeling months concisely:
#' 
#' * The first letter of each month
#' * Each January, the last 2 digits of the year
label_months <- function(sunspot_month) {
  month_letter <- sunspot_month %>% format("%b") %>% str_sub(1, 1)
  year_no_century <- sunspot_month %>% format("%y")
  paste0(
    month_letter,
    "\n",
    if_else(month(sunspot_month) == 1, year_no_century, "")
  )
}

#' The plot of the sun cycle from 1996 to 2008:
p <- 
  sunspots_reshaped %>% 
  filter(
    between(year(sunspot_month), 1996, 2008)
  ) %>% 
  ggplot() +
  aes(
    x = sunspot_month,
    y = sunspots
  ) +
  geom_area(
    fill  = "gray",
    color = "gray70"
  ) +
  scale_y_continuous(
    expand = expand_scale(mult = c(0, .05))
  ) +
  scale_x_date(
    date_breaks = "1 month",
    labels = label_months,
    expand = expand_scale(add = c(0, 0))
  ) +
  theme_minimal() +
  theme(
    axis.title.x     = element_blank(),
    panel.ontop      = TRUE,
    panel.grid.minor = element_blank(),
    panel.grid.major = element_line(color = "gray60")
  )

#' Totally busy as is:
p

#' Way prettier this way; `ggplot2` cycles through line colors!
p + 
  theme(
    panel.grid.major.x = element_line(
      color = c(
        "gray60", NA, NA,
        "white" , NA, NA,
        "white" , NA, NA,
        "white" , NA, NA
      )
    )
  )