# ggarchery: Flexible segment geoms with arrows for ggplot2


ggarchery is intended to extend [ggplot2](https://github.com/tidyverse/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 segment

First, let's generate some data that would be understood by [ggplot2](https://github.com/tidyverse/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))
```

<img src="man/figures/geom_segment_example.png" width="400"/>

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

<img src="man/figures/geom_arrowsegment_example1.png" width="400"/>

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))
```

<img src="man/figures/geom_arrowsegment_example2.png" width="400"/>

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))
```

<img src="man/figures/geom_arrowsegment_example3.png" width="400"/>

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))
```

<img src="man/figures/geom_arrowsegment_example4.png" width="400"/>

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))
```

<img src="man/figures/geom_arrowsegment_example5.png" width="400"/>

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))
```

<img src="man/figures/geom_arrowsegment_example6.png" width="400"/>

```
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))
```

<img src="man/figures/geom_arrowsegment_example7.png" width="400"/>


```
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))
```

<img src="man/figures/geom_arrowsegment_example8.png" width="400"/>



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))
```

<img src="man/figures/geom_arrowsegment_example9.png" width="400"/>

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))
```

<img src="man/figures/geom_arrowsegment_example10.png" width="400"/>

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))
```

<img src="man/figures/geom_arrowsegment_example11.png" width="400"/>

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))
```

<img src="man/figures/geom_arrowsegment_example12.png" width="400"/>

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")))
```
<img src="man/figures/annotate_example.png" width="800"/>


## `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")
```

<img src="man/figures/position_attractsegment_example1.png" width="400"/>

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")
```

<img src="man/figures/position_attractsegment_example2.png" width="400"/>

`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")
```

<img src="man/figures/position_attractsegment_example3.png" width="400"/>

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()
```

<img src="man/figures/position_attractsegment_example4.png" width="400"/>

(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()
```

<img src="man/figures/position_attractsegment_example5.png" width="400"/>

## Limitations

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.
