Proportionally sized arrows in ggplot

Question

Building on ggplot2's seals example, I'm trying to change the thickness of arrows so their overall size better reflects the data variable. I can specify length and thickness, but don't know how to change the size of the arrow-head. Very grateful for any suggestions.

require(ggplot2)
require(grid)

d = seals[sample(1:nrow(seals), 100),]
d$size = sqrt(sqrt(d$delta_long^2 + d$delta_lat^2))

ggplot(d, aes(x = long, y = lat, size = size)) +
  geom_segment(aes(xend = long + delta_long, yend = lat + delta_lat), arrow = arrow(length = unit(0.1,"cm")))

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Edit

Solution code:

ggplot(d, aes(x = long, y = lat, size = size)) +
  geom_segment(aes(xend = long + delta_long, yend = lat + delta_lat), 
               arrow = arrow(length = unit(d$size/3, "cm"), type='closed')) +
  scale_size(range = c(0, 2))

Answer 1

I can't say this is a complete solution to your problem, but at least it can be a start.

ggplot(d, aes(x = long, y = lat, size = size)) +
  geom_segment(aes(xend = long + delta_long, yend = lat + delta_lat), 
              arrow = arrow(length = unit(0.7, "cm"))) + 
  scale_size(range = c(1, 2))

My changes are minimal: bigger arrow heads and size scale. The upper limit on size scale is the most important if you aim to avoid overplotting.

From now on, it's probably a good idea to leave only the arrow heads, since the lines are not visible when the size is small. Here's a dirty hack for that:

 ggplot(d, aes(x = long, y = lat, size = size)) +
   geom_segment(aes(xend = long + delta_long/100, yend = lat + delta_lat/100), 
               arrow = arrow(length = unit(0.7,"cm"))) + 
   scale_size(range = c(1, 2))

Of course, it is crucial to keep an appropriate visual relation between big and small values! Otherwise, your plot may become misleading. But that depends on the data, so I cannot give you further advice. Sorry if that's an obvious point.

UPD: turns out, unit() function is vectorized, so it comes to the rescue!

ggplot(d, aes(x = long, y = lat, size = size)) +
  geom_segment(aes(xend = long + delta_long/100, yend = lat + delta_lat/100), 
              arrow = arrow(length = unit(d$size * 5,"cm"))) + 
  scale_size(range = c(1, 2))