Programmatically create a Scottish saltire using R

Create a digital flag of Scotland

Keywords

flag, ggplot2, Saint Andrew, saltire, Scotland

How to draw the flag of Saint Andrew?

The national flag of Scotland is called a saltire and has a blue background with a white diagonal cross. This is the flag of our patron saint, Saint Andrew.

I want to recreate this flag using R code.

Flag of Scotland

Code

library(ggplot2)
library(svglite)

# Flag colours, usually a shade of blue background with white diagonals
saltire_colour_background <- "#0065BD"
saltire_colour_lines <- "white"

# Flag dimensions ratio aspect ratio 5/3, will use mm as units
saltire_width <- 500 # 5cm
saltire_height <- 300 # 3cm
saltire_slope <- saltire_height / saltire_width

# "Due to a historical error, the unit of linewidth is roughly 0.75 mm."
# https://ggplot2.tidyverse.org/articles/ggplot2-specs.html
# line width on saltire should be between 1/3 and 1/5 width
# So 1/5 of saltire_width = 100, 100 * 0.75 = 75
saltire_line_width <- 75

plot_saltire <- ggplot() +
  # Draw the dark background of the flag
  geom_rect(
    aes(
      xmin = 0, xmax = saltire_width,
      ymin = 0, ymax = saltire_height
    ),
    fill = saltire_colour_background,
    color = saltire_colour_background
  ) +

  # Draw the diagonal from bottom left to top right
  geom_abline(
    intercept = 0,
    slope = saltire_slope,
    color = saltire_colour_lines,
    linewidth = saltire_line_width
  ) +

  # Draw the diagonal from top left to bottom right
  geom_abline(
    intercept = saltire_height,
    slope = -saltire_slope,
    color = saltire_colour_lines,
    linewidth = saltire_line_width
  ) +

  # Remove padding around x axis
  scale_x_continuous(expand = c(0, 0)) +

  # Remove padding around y axis
  scale_y_continuous(expand = c(0, 0)) +

  # 1:1 aspect ratio for physical scale
  coord_fixed() +

  # A completely empty theme.
  theme_void()

# Store a PNG version
ggsave("saltire.png",
  plot = plot_saltire,
  device = "png",
  dpi = 72,
  width = saltire_width,
  height = saltire_height,
  units = "mm"
)

# Store a SVG version
ggsave("saltire.svg",
  plot = plot_saltire,
  device = "svg",
  dpi = 72,
  width = saltire_width,
  height = saltire_height,
  units = "mm"
)

saltire PNG

saltire SVG