Segments and curves: geom_segment

Mix.install([
  {:giggly, path: Path.join(__DIR__, "..")},
  {:kino, "~> 0.19"}
  ])

About this notebook

This notebook follows the examples on ggplot2's reference page for geom_segment(), which covers geom_curve() too, in the order the page gives them. Each example shows ggplot2's code first and then Giggly's. An example Giggly cannot draw yet keeps its ggplot2 code and names what it is waiting for, so the page is also a list of what remains.

geom_segment() draws a straight line for each row, from (x, y) to (xend, yend), and geom_curve() draws a curved one; to join points across rows, ggplot2 has geom_path(), which Giggly has too (Giggly.Plot.geom_path/3). Giggly has neither of these yet.

A curve and a segment

b <- ggplot(mtcars, aes(wt, mpg)) +
  geom_point()

df <- data.frame(x1 = 2.62, x2 = 3.57, y1 = 21.0, y2 = 15.0)
b +
 geom_curve(aes(x = x1, y = y1, xend = x2, yend = y2, colour = "curve"), data = df) +
 geom_segment(aes(x = x1, y = y1, xend = x2, yend = y2, colour = "segment"), data = df)

Both layers draw a one-row table of their own, df, over the points of mtcars, and mapping colour to a constant names each layer in the legend.

Not in Giggly yet: geom_curve() and geom_segment(), and mapping xend and yend.

Curvature and arrows

b + geom_curve(aes(x = x1, y = y1, xend = x2, yend = y2), data = df, curvature = -0.2)
b + geom_curve(aes(x = x1, y = y1, xend = x2, yend = y2), data = df, curvature = 1)
b + geom_curve(
  aes(x = x1, y = y1, xend = x2, yend = y2),
  data = df,
  arrow = arrow(length = unit(0.03, "npc"))
)

A negative curvature bends the curve to the left, a positive one to the right, and 0 draws a straight line; the default is 0.5.

Not in Giggly yet: geom_curve() with its curvature and arrow options, arrow(), and mapping xend and yend.

Seal movements

if (requireNamespace('maps', quietly = TRUE)) {
ggplot(seals, aes(long, lat)) +
  geom_segment(aes(xend = long + delta_long, yend = lat + delta_lat),
    arrow = arrow(length = unit(0.1,"cm"))) +
  annotation_borders("state")
}

ggplot2's seals is a vector field of seals' movements: at each point of a grid of longitudes and latitudes, the change in longitude and latitude. annotation_borders("state") draws the outlines of the US states from the maps package.

Not in Giggly yet: geom_segment() and its arrow option, mapping xend and yend, annotation_borders() and the map data from the maps package it draws, and the seals dataset.

Line ends and joins

# Use lineend and linejoin to change the style of the segments
df2 <- expand.grid(
  lineend = c('round', 'butt', 'square'),
  linejoin = c('round', 'mitre', 'bevel'),
  stringsAsFactors = FALSE
)
df2 <- data.frame(df2, y = 1:9)
ggplot(df2, aes(x = 1, y = y, xend = 2, yend = y, label = paste(lineend, linejoin))) +
  geom_segment(
     lineend = df2$lineend, linejoin = df2$linejoin,
     size = 3, arrow = arrow(length = unit(0.3, "inches"))
  ) +
  geom_text(hjust = 'outside', nudge_x = -0.2) +
  xlim(0.5, 2)

R's expand.grid() makes a row for each combination of line end and line join, and each row's segment is drawn with its own line end and join, and labelled with their names.

Not in Giggly yet: geom_segment() with its lineend, linejoin and arrow options, and mapping xend and yend. ggplot2 knows no hjust called 'outside' and centres these labels, as Giggly does by default.

Spikes, as base R draws them

# You can also use geom_segment to recreate plot(type = "h") :
set.seed(1)
counts <- as.data.frame(table(x = rpois(100,5)))
counts$x <- as.numeric(as.character(counts$x))
with(counts, plot(x, Freq, type = "h", lwd = 10))

ggplot(counts, aes(x, Freq)) +
  geom_segment(aes(xend = x, yend = 0), linewidth = 10, lineend = "butt")

R's rpois() draws 100 random counts from a Poisson distribution with mean 5, and table() counts how often each occurs. Base R's plot(type = "h") draws a vertical line from 0 up to each frequency, and the ggplot() call draws the same with segments.

Not in Giggly yet: geom_segment() and its lineend option, and mapping xend and yend.