layoutGraph: A function to compute layouts of graph objects

Description

This is a wrapper to layout graph objects using arbitrary layout engines. The default engine (and so far the only implemented engine) is ATT's Graphviz.

Usage

layoutGraph(x, layoutFun = layoutGraphviz, ...)

Arguments

A graph object

layoutFun

A function that performs the graph layout and returns a graph object with all necessary rendering information

...

Further arguments that are passed to layoutFun

Value

An object inheriting from class graph

Details

Layout of a graph and its rendering are two separate processes. layoutGraph provides an API to use an arbitrary algorithm for the layout. This is archived by abstraction of the layout process into a separate function (layoutFun) with well-defined inputs and outputs. The only requirements on the layoutFun are to accept a graph object as input and to return a valid graph object with all the necessary rendering information stored in its renderInfo slot. This information comprises

for nodes:

nodeX, nodeY: the locations of the nodes, in the coordinate system defined by bbox (see below).

lWidth, rWidth

the width components of the nodes, lWidth+rWidth=total width.

height

the heights of the nodes.

labelX, labelY

node label locations.

labelJust

the justification of the node labels.

label

node label text.

shape

the node shape. Valid values are box, rectangle, ellipse, plaintext, circle and triangle.

for edges:

splines: representation of the edge splines as a list of BezierCurve objects.

labelX, labelY

edge label locations.

label

edge label text.

arrowhead, arrowtail

some of Graphviz's arrow shapes are supported. Currently they are: open, normal, dot, odot, box, obox, tee, diamond, odiamond and none. In addition, a user-defined function can be passed which needs to be able to deal with 4 arguments: A list of xy coordinates for the center of the arrowhead, and the graphical parameters col, lwd and lty.

direction

The edge direction. The special value both is used when reciprocrated edges are to be collapsed.

To indicate that this information has been added to the graph, the graph plotting function should also set the laidout flag in the graphData slot to TRUE and add the bounding box information (i.e., the coordinate system in which the graph is laid out) in the format of a two-by-two matrix as item bbox in the graphData slot.

AT&T's Graphviz is the default layout algorithm to use when layoutGraph is called without a specific layoutFun function. See agopen for details about how to tweak Graphviz and the valid arguments that can be passed on through .... The most common ones to set in this context might be layoutType, which controls the type of layout to compute and the nodeAttrs and edgeAttrs arguments, which control the fine-tuning of nodes and edges.

Examples

Run this code

set.seed(123)
V <- letters[1:5]
M <- 1:2
g1 <- randomGraph(V, M, 0.5)
edgemode(g1) <- "directed"
x <- layoutGraph(g1)
renderGraph(x)

## one of Graphviz's additional layout algorithms
x <- layoutGraph(g1, layoutType="neato")
renderGraph(x)

## some tweaks to Graphviz's node and edge attributes,
## including a user-defined arrowhead and node shape functions.
myArrows <- function(x, ...)
{
for(i in 1:4)
points(x,cex=i)
}

myNode <- function(x, col, fill, ...)
symbols(x=mean(x[,1]), y=mean(x[,2]), thermometers=cbind(.5, 1,
runif(1)), inches=0.5,
fg=col, bg=fill, add=TRUE)

eAtt <- list(arrowhead=c("a~b"=myArrows, "b~d"="odiamond", "d~e"="tee"))
nAtt <- list(shape=c(d="box", c="ellipse", a=myNode))
edgemode(g1) <- "directed"
x <- layoutGraph(g1, edgeAttrs=eAtt, nodeAttrs=nAtt, layoutType="neato")
renderGraph(x)

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