Plotting isolated nodes in igraph with graph_from_data_frame. Missing

Question

I'm working with data.frames with columns "from" and "to" and I would like to create network graphs from them.

For example:

mydata <- data.table(from=c("John", "John", "Jim", "Jesse"),
   to=c("John", "Jim", "Jack", NA))
mygraph <- graph_from_data_frame(d=mydata, directed=T)
plot(mygraph, vertex.label.dist=2) 

The presence of that NA produces an error.

If I just remove the NA row the lonely node is not plotted.

mydata <- data.table(from=c("John", "John", "Jim"),to=c("John", "Jim", "Jack"))
mygraph <- graph_from_data_frame(d=mydata, directed=T)
plot(mygraph, vertex.label.dist=2) 

I would like to get the same result than with:

g4 <- graph( c("John", "Jim", "Jim",  "Jack", "John", "John"), isolates=c("Jesse") )  
plot(g4,  vertex.label.dist=2) 

but working with two columns, from and to. How can I get the same result? When any of the "from" or "to" is NA then just plot the node without edges and without producing errors.

Answer 1

One way to get what you want is to leave out the single node, but then add it using add_vertices

library(igraph)
mydata <- data.frame(from=c("John", "John", "Jim"),
   to=c("John", "Jim", "Jack"))
mygraph <- graph_from_data_frame(d=mydata, directed=T)
mygraph = add_vertices(mygraph, 1, name="Jesse")
plot(mygraph, vertex.label.dist=2)