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rggraph

Controlling facet order in ggraph

I want to plot a graph using facets, where the edges vary between panels. The panels are automatically ordered alphabetically (as is customary in ggplot). A simple example:

library(igraph)
library(ggraph)

g <- make_empty_graph() + 
  vertices(1:2) +
  edges(1:2, 2:1, g = c('b', 'a'))

ggraph(g, 'kk') + 
  geom_edge_link(arrow = grid::arrow()) + 
  geom_node_label(aes(label = name)) +
  facet_edges(~g)

enter image description here

This is great, the node positions are presevered, but the edges differ depending on g.

However, I want to choose the order that facets appear. So in this case, first b then a, just like I ordered them while creating the graph above.

In ggplot one would change the order of the factor g. However, creating a layout does not show g:

create_layout(g, 'kk')

           x             y name ggraph.orig_index circular ggraph.index
1 -0.9021575 -1.410825e+00    1                 1    FALSE            1
2 -1.0000000  1.224606e-16    2                 2    FALSE            2

Changing the edge attributes into a factor manually, does change the ordering, but the labels are coerced to numeric:

g2 <- make_empty_graph() + 
  vertices(1:2) +
  edges(1:2, 2:1, g = factor(c('b', 'a'), levels = c('b', 'a')))

ggraph(g2, 'kk') + 
  geom_edge_link(arrow = grid::arrow()) + 
  geom_node_label(aes(label = name)) +
  facet_edges(~g)

enter image description here

How can I give custom ordering for the facets?

Solution

  • ggraph uses the same concept as ggplot2, that is, ordering of categorical data is done by using factors. Therefore you would control the facet order by encoding the ordering as levels in the facet variable.

    There is a slight problem though - igraph does not have very good support for factors so factors will often be dropped when assigned to node or edge data. I have submitted a PR to igraph that solves this, but in the meantime I would suggest you use tidygraph which works around this problem. Eventually tidygraph will be used behind everything in ggraph so you might just as well get on board🙂

    If you are determined to work directly in igraph and can't wait for the next version you can get factors into igraph by using vertex.attributes() in this manner:

    vertex.attributes(graph)$factor_attr <- value

    Or equivalently with edge.attributes