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rigraphggraph

How can I reduce the nodes in a ggraph arc graph?

I'm trying to create an arc graph showing relationships between nonprofits focusing on a subgraph centered on one of the nonprofits. There are so many nonprofits in this subgraph, I need to reduce the number of nodes in the arc graph to only focus on the strongest connections.

I've successfully filtered out edges below a weight of 50. But when I create the graph, the nodes are still remaining even though the edges have disappeared. How do I filter the unwanted nodes from the arc graph?

Here's my code, starting from the creation of the igraph object.

# Create an igraph object
NGO_igraph <- graph_from_data_frame(d = edges, vertices = nodes, directed = TRUE)

# Create a subgraph centered on a node
# Start by entering the node ID
nodes_of_interest <- c(48)

# Build the graph
selegoV <- ego(NGO_igraph, order=1, nodes = nodes_of_interest, mode = "all", mindist = 0)
selegoG <- induced_subgraph(NGO_igraph,unlist(selegoV))

# Reducing the graph based on edge weight
smaller <- delete.edges(selegoG, which(E(selegoG)$weight < 50))

# Plotting an arc graph 
ggraph(smaller, layout = "linear") + 
  geom_edge_arc(aes(width = weight), alpha = 0.8) + 
  scale_edge_width(range = c(0.2, 2)) +
  geom_node_text(aes(label = label)) +
  labs(edge_width = "Interactions") +
  theme_graph()

And here's the result I'm getting: enter image description here

Solution

  • If you are only interested in omitting zero degree vertices or isolates (meaning vertices which have no incoming or outgoing edge) you could simply use the following line:

    g <- induced.subgraph(g, degree(g) > 0)

    However, this will delete all isolates. So if you are for some reason set on specificly deleting those vertices connected by edges smaller than 50 (and exempt other 'special' isolates), then you will need to clearly identify which those are:

    special_vertex <- 1
v <- ends(g, which(E(g) < 50))
g <- delete.vertices(g, v[v != special_vertex])

    You could also skip the delete.edges part by considering the strength of a vertex:

    g <- induced.subgraph(g, strength(g) > 50)