In ggplot, how can I make a grid along x and y axis between categories?

Question

Below is the reproducible R script for plot without any panel.grid

require(ggplot2)
library(ggrepel)
# Create the data frame.
sales_data <- data.frame(
  emp_name = rep(c("Sam", "Dave", "John", "Harry", "Clark", "Kent", "Kenneth", "Richard", "Clement", "Toby", "Jonathan"), times = 5), 
  month = as.factor(rep(c("Jan", "Feb", "Mar", "Jan", "Feb", "Mar", "Jan", "Feb", "Mar", "Jan", "Jan"), times = 5)),
  dept_name = as.factor(rep(c("Production", "Services", "Support", "Support", "Services", "Production", "Production", "Support", "Support", "Support", "Production"), times = 5)), 
  revenue = rep(c(100, 200, 300, 400, 500, 600, 500, 400, 300, 200, 500), times = 5)
)

sales_data$month <- factor(sales_data$month, levels = c("Jan", "Feb", "Mar"))
month_vector <- levels(sales_data$month)
number_of_enteries <- nrow(sales_data)

sales_data$month <- as.integer(sales_data$month)

ggplot(sales_data, aes(x = month, y = dept_name)) +
  geom_raster(data = expand.grid(sales_data$month, sales_data$dept_name), 
            aes(x = Var1, y = Var2, width=1, height=1), fill = NA, col = 'gray50', lty = 1) + #default width and height is 1
  geom_point(aes(size = revenue, col = revenue), 
             shape = 16, position = position_jitter(seed = 0), show.legend = F) +
  geom_text_repel(aes(label = revenue), size=4, vjust = 1.6, position = position_jitter(seed = 0)) + #try with geom_text
  
  theme_bw() +
  theme(
    axis.title = element_blank(),
    axis.ticks = element_blank(),
    plot.background = element_blank(), 
    axis.line = element_blank(), 
    panel.border = element_blank(), 
    panel.grid = element_blank(),
    #panel.grid.major.y = element_line(colour = "red"),
    #panel.grid.major.x = element_line(colour = "red"),
    axis.text = element_text(colour = "blue", face = "plain", size =11)
  ) +
 
  scale_x_continuous(limits=c(0.5,3.5), expand = c(0,0), breaks = 1:length(month_vector), labels = month_vector)

Output plot is : Expected plot : I want to have tick lines along the x and y axis like shown in red line. I tried using panel.grid in theme but because of my custom added scale_x_continuous it is giving me unwanted major minor axes. Please uncomment line panel.grid.major.y = element_line(colour = "red"), to see the major grid.

Answer 1

Surprisingly, to the best of my knowledge, there doesn't seem to be a good way to get panel.grid lines between discrete categories. One way to get around this is to use hline and vline to add these lines:

To your ggplot code, add the following 3 lines:

# Remove extra whitespace from y-axis so lines are against the axis
scale_y_discrete(expand = c(0,0)) +
# Add straight lines at each factor level, shifted left/down so they're between values
geom_hline(yintercept = as.numeric(sales_data$dept_name) - 0.5) +
geom_vline(xintercept = as.numeric(sales_data$month) - 0.5)