Spiral flow tree

Computes a one-to-many flow map as an angle-restricted spiral tree (Verbeek, Buchin and Speckmann 2011). This is the recommended flow map layout; unlike tnss_tree it needs no dummy nodes or triangulation, keeps the real node positions fixed, produces a planar (crossing-free) tree with smooth, shallow-angle merges, and is tuned by a single parameter alpha.

Usage

flow_tree(object, xy, root, alpha = 40, n = 20, optimize = FALSE)

Arguments

object: a one-to-many flow network (star graph) as igraph/tbl_graph, with a weight edge attribute giving the flow
xy: coordinates of vertices
root: root node id of the flow
alpha: restricting angle in degrees (0-90). Smaller values bundle more tightly toward the root; typical values are 20-45.
n: number of points sampled per tree edge
optimize: logical. If TRUE, refine the tree with an approximate FLOWTREE optimization (Verbeek et al. 2011, section 5): join points stay fixed while edge interiors are relaxed for smoothness and to keep clear of node obstacles. This can slightly relax the strict alpha bound.

Value

data.frame with columns x, y, flow and edge (one smooth arc per tree edge)

References

Verbeek, Kevin, Kevin Buchin, and Bettina Speckmann. "Flow map layout via spiral trees." IEEE Transactions on Visualization and Computer Graphics 17, no. 12 (2011): 2536-2545.

Author

David Schoch

Examples

xy <- cbind(state.center$x, state.center$y)[!state.name %in% c("Alaska", "Hawaii"), ]
flow <- flow_tree(cali2010, xy, root = 4, alpha = 40)