---
title: "Layouts"
author: "Thomas Lin Pedersen"
date: "`r Sys.Date()`"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Layouts}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

In very short terms, a layout is the vertical and horizontal placement of nodes
when plotting a particular graph structure. Conversely, a layout algorithm is an
algorithm that takes in a graph structure (and potentially some additional 
parameters) and return the vertical and horizontal position of the nodes. Often,
when people think of network visualizations, they think of node-edge diagrams
where strongly connected nodes are attempted to be plotted in close proximity.
Layouts can be a lot of other things too though --- e.g. hive plots and 
treemaps. One of the driving factors behind `ggraph` has been to develop an API
where any type of visual representation of graph structures is supported. In 
order to achieve this we first need a flexible way of defining the layout...

## The ggraph() and create_layout() functions
As the layout is a global specification of the spatial position of the nodes it
spans all layers in the plot and should thus be defined outside of calls to 
geoms or stats. In `ggraph` it is often done as part of the plot initialization
using `ggraph()` --- a function equivalent in intent to `ggplot()`. As a minimum
`ggraph()` must be passed a graph object supported by `ggraph`:

```{r, message=FALSE}
library(ggraph)
library(tidygraph)

set_graph_style(plot_margin = margin(1,1,1,1))
graph <- as_tbl_graph(highschool)

# Not specifying the layout - defaults to "auto"
ggraph(graph) + 
  geom_edge_link(aes(colour = factor(year))) + 
  geom_node_point()
```

Not specifying a layout will make `ggraph` pick one for you. This is only 
intended to get quickly up and running. The choice of layout should be 
deliberate on the part of the user as it will have a great effect on what the 
end result will communicate. From now on all calls to `ggraph()` will contain a
specification of the layout:

```{r}
ggraph(graph, layout = 'kk') + 
  geom_edge_link(aes(colour = factor(year))) + 
  geom_node_point()
```

If the layout algorithm accepts additional parameters (most do), they can be
supplied in the call to `ggraph()` as well:

```{r}
ggraph(graph, layout = 'kk', maxiter = 100) + 
  geom_edge_link(aes(colour = factor(year))) + 
  geom_node_point()
```

If any layout parameters refers to node or edge variables they must be supplied
as unquoted expression (like inside `aes()` and `tidyverse` verbs)

In addition to specifying the layout during plot creation it can also happen 
separately using `create_layout()`. This function takes the same arguments as 
`ggraph()` but returns a `layout_ggraph` object that can later be used in place
of a graph structure in ggraph call:

```{r}
layout <- create_layout(graph, layout = 'eigen')
ggraph(layout) + 
  geom_edge_link(aes(colour = factor(year))) + 
  geom_node_point()
```

Examining the return of `create_layout()` we see that it is really just a 
`data.frame` of node positions and (possible) attributes. Furthermore the 
original graph object along with other relevant information is passed along as
attributes:

```{r}
head(layout)
```

```{r}
attributes(layout)
```

As it is just a `data.frame` it means that any standard `ggplot2` call will work
by addressing the nodes. Still, use of the `geom_node_*()` family provided by 
`ggraph` is encouraged as it makes it explicit which part of the data structure
is being worked with.

## Adding support for new data sources
Out of the box `ggraph` supports `tbl_graph` objects from tidygraph natively. 
Any other type of object will be attempted to be coerced to a `tbl_graph` object
automatically. Tidygraph provide conversions for most known graph structure in R
so almost any data type is supported by ggraph by extension. If there
is wish for support for additional classes this can be achieved by providing a
`as_tbl_graph()` method for the class. If you do this, consider submitting the
method to tidygraph so others can benefit from your work.

## Layouts abound
There's a lot of different layouts in `ggraph` --- All layouts from the 
graphlayouts and igraph packages are available, an ggraph itself also provide
some of the more specialised layouts itself. All in all ggraph provides well 
above 20 different layouts to choose from, far more than we can cover in this 
text. I urge you to explore the different layout types. Blindly running along 
with the default layouts is a sad but common mistake in network visualisation 
that can cloud or distort the insight the network might hold. If ggraph lacks 
the needed layout it is always possible to supply your own layout function that
takes a tbl_graph object and returns a data.frame of node positions, or supply
the positions directly by passing a matrix or data.frame to the layout argument.

### A note on circularity
Some layouts can be shown effectively both in a standard Cartesian projection as
well as in a polar projection. The standard approach in `ggplot2` has been to 
change the coordinate system with the addition of e.g. `coord_polar()`. This
approach --- while consistent with the grammar --- is not optimal for `ggraph`
as it does not allow layers to decide how to respond to circularity. The prime
example of this is trying to draw straight lines in a plot using 
`coord_polar()`. Instead circularity is part of the layout specification and
gets communicated to the layers with the `circular` column in the data, allowing
each layer to respond appropriately. Sometimes standard and circular 
representations of the same layout get used so often that they get different
names. In `ggraph` they'll have the same name and only differ in whether or not
`circular` is set to `TRUE`:

```{r}
# An arc diagram
ggraph(graph, layout = 'linear') + 
  geom_edge_arc(aes(colour = factor(year)))
```

```{r}
# A coord diagram
ggraph(graph, layout = 'linear', circular = TRUE) + 
  geom_edge_arc(aes(colour = factor(year))) + 
  coord_fixed()
```

```{r}
graph <- tbl_graph(flare$vertices, flare$edges)
# An icicle plot
ggraph(graph, 'partition') + 
  geom_node_tile(aes(fill = depth), size = 0.25)
```

```{r}
# A sunburst plot
ggraph(graph, 'partition', circular = TRUE) + 
  geom_node_arc_bar(aes(fill = depth), size = 0.25) + 
  coord_fixed()
```

Not every layout has a meaningful circular representation in which cases the 
`circular` argument will be ignored.

### Node-edge diagram layouts
Both `graphlayout` and `igraph` provides a range of different layout algorithms 
for classic node-edge diagrams (colloquially referred to as hairballs). Some of 
these are incredibly simple such as *randomly*, *grid*, *circle*, and *star*, 
while others tries to optimize the position of nodes based on different 
characteristics of the graph. There is no such thing as "the best layout 
algorithm" as algorithms have been optimized for different scenarios. Experiment 
with the choices at hand and remember to take the end result with a grain of 
salt, as it is just one of a range of possible "optimal node position" results. 
Below is a sample of some of the layouts available through `igraph` applied to 
the highschool graph.

```{r, fig.show='hold', results='hide'}
graph <- as_tbl_graph(highschool) %>% 
  mutate(degree = centrality_degree())
lapply(c('stress', 'fr', 'lgl', 'graphopt'), function(layout) {
  ggraph(graph, layout = layout) + 
    geom_edge_link(aes(colour = factor(year)), show.legend = FALSE) +
    geom_node_point() + 
    labs(caption = paste0('Layout: ', layout))
})
```

### Hive plots
A hive plot, while still technically a node-edge diagram, is a bit different
from the rest as it uses information pertaining to the nodes, rather than the
connection information in the graph. This means that hive plots, to a certain
extent are more interpretable as well as less vulnerable to small changes in the
graph structure. They are less common though, so use will often require some 
additional explanation.

```{r}
graph <- graph %>% 
  mutate(friends = ifelse(
    centrality_degree(mode = 'in') < 5, 'few',
    ifelse(centrality_degree(mode = 'in') >= 15, 'many', 'medium')
  ))
ggraph(graph, 'hive', axis = friends, sort.by = degree) + 
  geom_edge_hive(aes(colour = factor(year))) + 
  geom_axis_hive(aes(colour = friends), size = 2, label = FALSE) + 
  coord_fixed()
```

### Focal layouts
Some layouts can put focus on a single node or a group of nodes by defining all
other positions relative to that. An example of this is the `focus` layout, but
the `centrality` layout is very akin to it:

```{r}
ggraph(graph, 'focus', focus = node_is_center()) + 
  ggforce::geom_circle(aes(x0 = 0, y0 = 0, r = r), data.frame(r = 1:5), colour = 'grey') + 
  geom_edge_link() + 
  geom_node_point() + 
  coord_fixed()
```

### Hierarchical layouts
Trees and hierarchies are an important subset of graph structures, and `ggraph`
provides a range of layouts optimized for their visual representation. Some of
these use enclosure and position rather than edges to communicate relations
(e.g. treemaps and circle packing). Still, these layouts can just as well be 
used for drawing edges if you wish to:

```{r}
graph <- tbl_graph(flare$vertices, flare$edges)
set.seed(1)
ggraph(graph, 'circlepack', weight = size) + 
  geom_node_circle(aes(fill = depth), size = 0.25, n = 50) + 
  coord_fixed()
```

```{r}
set.seed(1)
ggraph(graph, 'circlepack', weight = size) + 
  geom_edge_link() + 
  geom_node_point(aes(colour = depth)) +
  coord_fixed()
```

```{r}
ggraph(graph, 'treemap', weight = size) + 
  geom_node_tile(aes(fill = depth), size = 0.25)
```

```{r}
ggraph(graph, 'treemap', weight = size) + 
  geom_edge_link() + 
  geom_node_point(aes(colour = depth))
```

The most recognized tree plot is probably dendrograms though. If nothing else is
stated the height of each node is calculated based on the distance to its 
farthest sibling (the tree layout, on the other hand, puts all nodes at a 
certain depth at the same level):

```{r}
ggraph(graph, 'tree') + 
  geom_edge_diagonal()
```

The height of each branch point can be set to a variable --- e.g. the height 
provided by hclust and dendrogram objects:

```{r}
dendrogram <- hclust(dist(iris[, 1:4]))
ggraph(dendrogram, 'dendrogram', height = height) + 
  geom_edge_elbow()
```

Dendrograms are one of the layouts that are amenable for circular 
transformations, which can be effective in giving more space at the leafs of the
tree at the expense of the space given to the root:

```{r}
ggraph(dendrogram, 'dendrogram', circular = TRUE) + 
  geom_edge_elbow() + 
  coord_fixed()
```

A type of trees known especially in phylogeny is unrooted trees, where no node
is considered the root. Often a dendrogram layout will not be faithful as it
implicitly position a node at the root. To avoid that you can use the `unrooted` 
layout instead.

```{r}
ggraph(dendrogram, 'unrooted') + 
  geom_edge_link()
```

Often unrooted trees have a branch length attached - this can be passed to both
the `dendrogram` and `unrooted` layout to determine the length of each edge.

### Matrix layouts
Many node-edge diagram layouts suffer from poor scalability, where edges will 
eventually begin to overlap to the extend that the plot becomes unintellible. 
One way to combat this is by only plotting subsets of the larger plot, but
another approach is to choose a layout that avoids overlapping edges altogether.
The matrix layout places each node on a diagonal and draws edges by drawing 
points or tiles at the intersection of the vertical and horizontal position of
its terminal nodes. Using matrix layouts efficiently requires that you begin to
recognise the specific patterns that different network topologies gives rise to.
Further, it is important to recognise the large effect that the node order has 
on the look of the matrix layout:

```{r}
graph <- create_notable('zachary')
ggraph(graph, 'matrix', sort.by = node_rank_leafsort()) + 
  geom_edge_point(mirror = TRUE) + 
  coord_fixed()
```

```{r}
ggraph(graph, 'matrix', sort.by = node_rank_spectral()) + 
  geom_edge_point(mirror = TRUE) + 
  coord_fixed()
```

### Fabric layouts
Another special layout that promises scalability is the biofabric layout (here
named `fabric` to avoid it being ignored for non-biological networks). The 
fabric layout is special in that it positions nodes evenly spaced on the y-axis
and then draws edges as vertical (and by extension, parallel) lines evenly 
separated as well, connecting the nodes. Nodes are drawn as horizontal lines 
spanning the extent of the edges that departs from it. As with matrix layouts
the node ordering have a huge impact on the final look and interpreting the plot
may take some getting used to.

```{r}
ggraph(graph, 'fabric', sort.by = node_rank_fabric()) + 
  geom_node_range(colour = 'grey') + 
  geom_edge_span(end_shape = 'square') + 
  coord_fixed()
```

Fabric layouts allow something called shadow edges where all edges are 
duplicated to make it easier to follow all edges originating from each node.

```{r}
ggraph(graph, 'fabric', sort.by = node_rank_fabric(), shadow.edges =TRUE) + 
  geom_node_range(colour = 'grey') + 
  geom_edge_span(aes(filter = shadow_edge), colour ='lightblue' , end_shape = 'square') + 
  geom_edge_span(aes(filter = !shadow_edge), end_shape = 'square') + 
  coord_fixed()
```

## Want more?
Check out the other vignettes for more information on how to draw 
[nodes](Nodes.html) and [edges](Edges.html)...
