Add metro layout

NAMESPACE

@@ -299,10 +299,12 @@ importFrom(ggforce,radial_trans)
 importFrom(ggrepel,GeomLabelRepel)
 importFrom(ggrepel,GeomTextRepel)
 importFrom(graphlayouts,layout_as_backbone)
+importFrom(graphlayouts,layout_as_metromap)
 importFrom(graphlayouts,layout_with_centrality)
 importFrom(graphlayouts,layout_with_centrality_group)
 importFrom(graphlayouts,layout_with_constrained_stress)
 importFrom(graphlayouts,layout_with_eigen)
+importFrom(graphlayouts,layout_with_fixed_coords)
 importFrom(graphlayouts,layout_with_focus)
 importFrom(graphlayouts,layout_with_focus_group)
 importFrom(graphlayouts,layout_with_pmds)

NEWS.md

@@ -30,6 +30,7 @@
 * Added `geom_edge_bundle_force()`, `geom_edge_bundle_path()`, and 
   `geom_edge_bundle_minimal()` (+ variants) to provide support for edge bundling 
   (#267)
+* Add "metro" layout from graphlayouts for metroline like layouts
 
 # ggraph 2.1.0
 

R/layout_metro.R

@@ -0,0 +1,66 @@
+#' Place nodes according to the standard design of metro maps
+#'
+#' This layouttries to optimise the placement of nodes so that they adhere to
+#' the classic look of metro maps. As such it optimizes on the distribution of
+#' incident edge angles, conformity of edge length, and edge angles in multiples
+#' of 45 degrees. As it works as a refinement of an already existing layout
+#' (the physical location of metro stations) it requires an a priori node
+#' location. Due to it's purpose it probably works best with planar graphs.
+#'
+#' @param graph A tbl_graph object
+#' @param x,y The start location of the nodes
+#' @param length Desired multiple of grid point spacing. (`length * grid_space`
+#' determines desired edge length)
+#' @param grid_space The distance between consecitive grid points
+#' @param optimization_weights The relative weight to be placed on the 5
+#' criteria during optimization as a numeric vector of length 4. The criteria
+#' are:
+#' * `Angular Resolution Criterion`: The angles of incident edges at each
+#'   station should be maximized, because if there is only a small angle between
+#'   any two adjacent edges, then it can become difficult to distinguish between
+#'   them.
+#' * `Edge Length Criterion`: The edge lengths across the whole map should be
+#'   approximately equal to ensure regular spacing between stations. It is based
+#'   on the preferred multiple, l, of the grid spacing, g. The purpose of the
+#'   criterion is to penalize edges that are longer than or shorter than lg.
+#' * `Balanced Edge Length Criterion`: The length of edges incident to a
+#'   particular station should be similar.
+#' * `Line Straightness Criterion`: (not yet implemented) Edges that form part
+#'   of a line should, where possible, be co-linear either side of each station
+#'   that the line passes through.
+#' * `Octiinearity Criterion`: Each edge should be drawn horizontally,
+#'   vertically, or diagonally at 45 degree, so we penalize edges that are not
+#'   at a desired angle.
+#' If `NULL` all criteria are given equal weight.
+#' @param max_movement Number of grid points a station can move away rom its
+#' original position
+#' @param circular ignored
+#'
+#' @return A data.frame with the columns `x`, `y`, `circular` as
+#' well as any information stored as node variables in the tbl_graph object.
+#'
+#' @references
+#' Stott, J., Rodgers, P., Martinez-Ovando, J. C., and Walker, S. G. (2011).
+#' *Automatic metro map layout using multicriteria optimization* In IEEE Trans
+#' Vis Comput Graph 17(1) pp. 101-114. https://doi.org/10.1109/tvcg.2010.24
+#'
+#' @family layout_tbl_graph_*
+#'
+#' @author The underlying algorithm is implemented in the graphlayouts package
+#' by David Schoch
+#'
+#' @importFrom graphlayouts layout_as_metromap
+#'
+layout_tbl_graph_metro <- function(graph, x, y, length = 2, grid_space = 0.0025,
+                                   optimization_weights = NULL, max_movement = 5,
+                                   circular = FALSE) {
+  x <- eval_tidy(enquo(x), .N())
+  y <- eval_tidy(enquo(y), .N())
+  if (is.null(x) || is.null(y)) {
+    cli::cli_abort("The metro layout must have a prior location of nodes to start with")
+  }
+  if (is.null(optimization_weights)) optimization_weights <- rep(1, 5)
+  xy <- layout_as_metromap(graph, xy = cbind(x, y), l = length, gr = grid_space, w = optimization_weights, bsize = max_movement)
+  nodes <- data_frame0(x = xy[,1], y = xy[,2], circular = FALSE)
+  combine_layout_nodes(nodes, as_tibble(graph, active = 'nodes'))
+}