R: how do I draw a line with multiple arrows in it?

Question

I was wondering if anyone could help me plot lines in R with multiple arrows in them, like this:

--->--->--->--->

Thanks in advance!

Answer 1

I thought I would add a base R solution here that calculates the arrow heads as segments. There are versions to create a new plot or add arrows to an existing plot. Longer segments are split into shorter ones:

add_arrows <- function(x, y, ...)
{
  # Ensure equal lengthed vectors
  min_len <- min(length(x), length(y))
  x  <- x[seq(min_len)]
  y  <- y[seq(min_len)]

  # Calculate y:x ratio
  yx_ratio <- 3*(max(y) - min(y)) / (max(x) - min(x))

  # Create start and end points
  x1 <- x[-length(x)]
  y1 <- y[-length(y)]
  x2 <- x[-1]
  y2 <- y[-1]

  # Length and angle of line segments
  theta <- atan((y2 - y1)/(x2 - x1))
  seg_lengths <- sqrt((x2 - x1)^2 + (y2 - y1)^2)

  # Break long segments into shorter segments
  long <- which(seg_lengths > (2 * min(seg_lengths)))
  if(length(long) > 0)
  {
    n_segs <- floor(seg_lengths[long] / (1.5 * min(seg_lengths)))

    for(i in seq_along(long))
    {
      xs <- x1[long[i]] + (0:n_segs[i]) * (x2[long[i]] - x1[long[i]]) / n_segs[i]
      ys <- y1[long[i]] + (0:n_segs[i]) * (y2[long[i]] - y1[long[i]]) / n_segs[i]
      x1 <- c(x1, xs[-length(xs)])
      x2 <- c(x2, xs[-1])
      y1 <- c(y1, ys[-length(ys)])
      y2 <- c(y2, ys[-1])
      theta <- c(theta, rep(theta[long[i]], length(xs) - 1))
    }

    x1 <- x1[-long]; x2 <- x2[-long]; y1 <- y1[-long]; y2 <- y2[-long];
    theta <- theta[-long]
  }
  # Arrow head length and width
  len <- sqrt((max(y) - min(y))^2 + (max(x) - min(x))^2)/ 20
  wid <- len * 0.1


  # Calculate arrow heads
  zx  <- x2 - len * cos(theta) * sign(x2 - x1)
  zy  <- y2 - len * sin(theta) * sign(x2 - x1)
  ax1 <- zx + wid * sin(theta) * sign(x2 - x1)
  ax2 <- zx - wid * sin(theta) * sign(x2 - x1)
  ay1 <- zy - wid * cos(theta) * sign(x2 - x1) * yx_ratio
  ay2 <- zy + wid * cos(theta) * sign(x2 - x1) * yx_ratio

  df <- data.frame(x1  = x1 + 0.1 * (x2 - x1),
                   y1  = y1 + 0.1 * (y2 - y1),
                   x2  = x2 - 0.5 * len * cos(theta) * sign(x2 - x1),
                   y2  = y2 - 0.5 * len * sin(theta) * sign(x2 - x1),
                   ax1 = ax1, ay1 = ay1, ax2 = ax2, ay2 = ay2)

  # Draw segments
  segments(df$x1, df$y1, df$x2,  df$y2,  col = "red", lwd = 2)
  segments(df$x2, df$y2, df$ax1, df$ay1, col = "red", lwd = 2)
  segments(df$x2, df$y2, df$ax2, df$ay2, col = "red", lwd = 2)
}

This allows you to plot arrows as the basis of your plot:

plot_arrows <- function(x, y, ...)
{
  plot(x, y, col = "white", ...)
  add_arrows(x, y, ...)
}

So now you can do:

set.seed(69)

x <- 1:10
y <- sample(10)
plot(x, y)
add_arrows(x, y)

^{Created on 2020-02-29 by the reprex package (v0.3.0)}