layoutGraph: A function to compute layouts of graph objects
In Rgraphviz: Provides plotting capabilities for R graph objects

Description Usage Arguments Details Value Note Author(s) See Also Examples

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.

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

`x`	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`

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.

An object inheriting from class graph

Please note that the layout needs to be recomputed whenever attributes are changed which are bound to affect the position of nodes or edges. This is for instance the case for the arrowhead and arrowtail parameters.

Florian Hahne, Deepayan Sarkar

renderGraph, graph.par, nodeRenderInfo, edgeRenderInfo, agopen,

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)