ggarchery is intended to extend ggplot2’s handling of segments with arrowheads. At present it contains one geom and one position adjustment.

`geom_arrowsegment()`

allows placement of one or more arrowheads at any point on a
segmentFirst, let’s generate some data that would be understood by ggplot2’s normal
`geom_segment()`

:

```
library(tidyverse)
library(ggarchery)
tbl <- tibble(x = c(0.1, 0.2), xend = c(0.1, 0.8), y = c(0.1, 0.5), yend = c(0.7, 0.9))
```

The default behaviour of `geom_arrowsegment()`

mimics that
of `geom_segment(arrow = arrow())`

```
ggplot(tbl) +
geom_segment(aes(x = x, xend = xend, y = y, yend = yend), arrow = arrow()) +
xlim(c(0,1)) +
ylim(c(0,1))
```

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend)) +
xlim(c(0,1)) +
ylim(c(0,1))
```

The `arrows`

parameter of `geom_arrowsegment()`

also behaves exactly like the `arrow`

parameter of
`geom_segment`

, as a call to `grid::arrow()`

:

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend),
arrows = arrow(type = 'closed')) +
xlim(c(0,1)) +
ylim(c(0,1))
```

Now for the interesting bit. Suppose that we would like the arrowhead
to appear at the midpoint of the segment, rather than the end. This can
be done by specifying `arrow_positions = 0.5`

.

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend), arrow_positions = 0.5) +
xlim(c(0,1)) +
ylim(c(0,1))
```

Control of the arrow segment works as before:

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend),
arrow_positions = 0.5,
arrows = arrow(type = 'closed')) +
xlim(c(0,1)) +
ylim(c(0,1))
```

Other aesthetics also work as you would hope. There is a subtle
difference in the legend as displayed by `geom_segment`

and
`geom_arrowsegment`

, however:

```
tbl <- tbl %>% mutate(col = c("A", "B"))
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend, col = col),
arrow_positions = 0.5) +
xlim(c(0,1)) +
ylim(c(0,1))
```

Another key way that `geom_arrowsegment`

differs from
`geom_segment`

is that it has a working `fill`

aesthetic. This is only visible if the arrowhead is closed. Note that it
must be specified as `fill`

even if you want it to simply
match the `colour`

aesthetic, another difference in behaviour
from `geom_segment`

.

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend, fill = col),
arrow_positions = 0.5, arrows = arrow(type = "closed")) +
xlim(c(0,1)) +
ylim(c(0,1))
```

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend, col = col),
arrow_positions = 0.5, arrows = arrow(type = "closed")) +
xlim(c(0,1)) +
ylim(c(0,1))
```

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend,
fill = col, col = col),
arrow_positions = 0.5,
arrows = arrow(type = "closed")) +
xlim(c(0,1)) +
ylim(c(0,1))
```

You can also define multiple arrowheads by making
`arrow_positions`

a vector of length greater than 1. All
values are expected to fall between 0 and 1, and not be exactly 0:

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend),
arrow_positions = c(0.25, 0.75)) +
xlim(c(0,1)) +
ylim(c(0,1))
```

If one value is 1, then the final arrowhead appears at the end:

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend),
arrow_positions = c(0.25, 1)) +
xlim(c(0,1)) +
ylim(c(0,1))
```

The look of each arrow can also be controlled separately by making
`arrows`

a list:

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend),
arrow_positions = c(0.25, 1),
arrows = list(arrow(angle = 10),
arrow(type = 'closed'))) +
xlim(c(0,1)) +
ylim(c(0,1))
```

The `arrow_fills`

option also mimics
`arrow.fill`

of `geom_segment()`

but can be a
vector. As with a specification of `fill`

outside the
aesthetics, this takes precedence over a `fill`

aesthetic.

```
ggplot(tbl) +
geom_arrowsegment(aes(x = x, xend = xend, y = y, yend = yend),
arrow_positions = c(0.25, 1),
arrow_fills = c("indianred3", "dodgerblue3"),
arrows = arrow(type = "closed")) +
xlim(c(0,1)) +
ylim(c(0,1))
```

Finally, the geom can be used as an annotation:

```
ggplot(mtcars) +
geom_point(aes(x = disp, y=hp)) +
annotate(geom = "arrowsegment",
x = 170,
y=200,
xend = 145,
yend = 175,
arrow_positions = 0.6,
arrows = arrow(type = "closed", length = unit(0.1, "inches")))
```

`position_attractsegment()`

allows you to automatically shave the ends of arrow segments`position_attractsegment()`

is intended to solve the
following problem. Suppose you have nicely laid out a set of labelled
points:

```
pt.tbl <- tibble(x = c(0.25, 0.5, 0.75), y = c(0.25, 0.5, 0.75), labels = c("A", "B", "C"))
ggplot(pt.tbl) +
geom_point(aes(x,y, fill = labels), size =6, shape = 21) +
geom_text(aes(x,y, label = labels)) +
xlim(c(0, 1)) +
ylim(c(0, 1)) +
scale_fill_discrete(guide = "none")
```

If you wish to connect these points using `geom_segment()`

with an arrow, the output is a little ugly, as the lines intersect the
points:

```
sg.tbl <- tibble(x = c(0.25, 0.5), y = c(0.25, 0.5), xend = c(0.5, 0.75), yend = c(0.5, 0.75))
ggplot(pt.tbl) +
geom_point(aes(x,y, fill = labels), size =6, shape = 21) +
geom_text(aes(x,y, label = labels)) +
geom_segment(data = sg.tbl,
aes(x = x, xend = xend, y = y, yend = yend),
arrow = arrow()) +
xlim(c(0, 1)) +
ylim(c(0, 1)) +
scale_fill_discrete(guide = "none")
```

`position_attractsegment()`

works by shortening the
segment at the start and the end (“attracting” the start and end points
towards each other). It can do this in two ways, as determined by the
`type_shave`

option. If
`type_shave = "proportion"`

(the default), then it takes the
proportions `start_shave`

and `end_shave`

away:

```
ggplot(pt.tbl) +
geom_point(aes(x,y, fill = labels), size =6, shape = 21) +
geom_text(aes(x,y, label = labels)) +
geom_segment(data = sg.tbl,
aes(x = x, xend = xend, y = y, yend = yend),
arrow = arrow(),
position = position_attractsegment(start_shave = 0.1,
end_shave = 0.1)) +
xlim(c(0, 1)) +
ylim(c(0, 1)) +
scale_fill_discrete(guide = "none")
```

Alternatively, if `type_shave = "distance"`

then the
amount removed is in graph units. This allows for finer control, but has
strange effects if the dimensions of the x and y axes are not the same
and is only really recommended in combination with
`coord_fixed()`

.

```
ggplot(pt.tbl)+
geom_segment(data = sg.tbl, aes(x = x, xend = xend, y = y, yend = yend), arrow = arrow(),
position = position_attractsegment(start_shave = 0,
end_shave = 0.05,
type_shave = "distance")) +
geom_point(aes(x,y, fill = labels), size =6, shape = 21) +
geom_text(aes(x,y, label = labels)) +
xlim(c(0, 1)) +
ylim(c(0, 1)) +
scale_fill_discrete(guide = "none") +
coord_fixed()
```

(Note here we shaved only the end of the segment, and drew the segment first.)

`geom_arrowsegment()`

and
`position_attractsegment()`

can naturally be used in
combination:

```
ggplot(pt.tbl)+
geom_arrowsegment(data = sg.tbl, aes(x = x, xend = xend, y = y, yend = yend),
arrow_positions = c(0.5, 0.6),
arrows = arrow(length = unit(0.1, "inches")),
position = position_attractsegment(start_shave = 0.05,
end_shave = 0.05,
type_shave = "distance")) +
geom_point(aes(x,y, fill = labels), size =6, shape = 21) +
geom_text(aes(x,y, label = labels)) +
xlim(c(0, 1)) +
ylim(c(0, 1)) +
scale_fill_discrete(guide = "none") +
coord_fixed()
```

Current these replace only `geom_segment()`

and work only
for linear coordinate systems. I would like to extend to
`geom_curve()`

but the intricacies of
`grid::curveGrob()`

make that much more complicated. Also the
fact that the specified arrow position corresponds to the arrowhead tip
can make lines look a little lopsided; it would be much better if the
specified point was the midpoint of the arrowhead instead, but this is
very difficult using `grid::arrow()`

because of the units
specification. To do this I would probably need to draw my own
arrowheads. Maybe one day…

Next on my to-do list is extending to the `geom_line()`

and `geom_path()`

parameterisations.