This function returns another function that can extract edges from a
ggraph_layout object. The functionality of the returned function is decided
by the arguments to get_edges. The need for get_edges is mainly to
pass to the data argument of the different geom_edge_*
functions in order to present them with the right kind of data. In general
each geom_edge_* has the default set correctly so there is only need
to modify the data argument if parallel edges should be collapsed.
get_edges(format = "short", collapse = "none", ...)Either 'short' (the default) or 'long'. See
details for a descriptions of the differences
Either 'none' (the default), 'all' or
'direction'. Specifies whether parallel edges should be merged. See
details for more information
Additional data that will be cbind'ed together with the returned edge data.
A data.frame with columns dependent on format as well as the graph
type. In addition to the columns discussed in the details section,
the data.frame will always contain the columns from, to and
circular, the two former giving the indexes of the start and end node
and the latter if the layout is circular (needed for correct formatting of
some geom_edge_*). The graph dependent information is:
A label column will hold the value of the
edgetext attribute. In addition any value stored in the
edgePar attribute will be added. Lastly a direction column
will hold the relative position between the start and end nodes (needed for
correct formatting of geom_edge_elbow).
All edge attributes of the original graph object is added as columns to the data.frame
There are two types of return formats possible for the result of the returned function:
In this format each edge is described in one line in the
format expected for geom_segment, that is, the start
node position is encoded in the x and y column and the end
node position is encoded in the xend and yend column. If node
parameters are added to the edge the name of the parameters will be
prefixed with node1. for the start node and node2. for the
end node.
In this format each edge consists of two rows with matching
edge.id value. The start and end position are both encoded in the
x and y column. The relative position of the rows determines
which is the start and end node, the first occuring being the start node.
If node parameters are added to the edge data the name of the parameters
will be prefixed with node..
Node parameters are automatically added so it is possible to format edge aesthetics according to start or end node parameters, or interpolate edge aesthetics between start and end node parameters. Node parameters will be prefixed to avoid name clash with edge parameters. The prefix depends on the format (see above).
If the graph is not simple (it contains at most one edge between each node pair) it can be collapsed so either all edges between two nodes or all edges of the same direction between two nodes are merged. The edge parameters are taken from the first occuring edge, so if some more sophisticated summary is needed it is suggested that the graph be tidied up before plotting with ggraph.