Removing arrows and make the line hearver igraph

Question

In this graph:

The input data are:

df <- structure(list(book1 = structure(1:4, .Label = c("refid1", "refid2", 
"refid3", "refid7"), class = "factor"), book2 = structure(c(2L, 
3L, 3L, 1L), .Label = c("", "refid1", "refid3"), class = "factor"), 
    book3 = structure(c(2L, 3L, 4L, 1L), .Label = c("", "refid1", 
    "refid2", "refid4"), class = "factor"), book4 = structure(c(2L, 
    3L, 4L, 1L), .Label = c("", "refid1", "refid3", "refid4"), class = "factor"), 
    book5 = structure(c(2L, 3L, 3L, 1L), .Label = c("", "refid2", 
    "refid6"), class = "factor"), book6 = structure(c(3L, 2L, 
    1L, 1L), .Label = c("", "refid1", "refid2"), class = "factor"), 
    book7 = structure(c(2L, 3L, 1L, 1L), .Label = c("", "refid1", 
    "refid5"), class = "factor")), .Names = c("book1", "book2", 
"book3", "book4", "book5", "book6", "book7"), class = "data.frame", row.names = c(NA, 
-4L))

and the graph plot comes from this:

library(dplyr)
library(igraph)
library(tidyr)


g<- df %>% 
  gather(key = "From", "To", na.rm = TRUE) %>% 
  graph_from_data_frame()

plot(g)

How is it possible to remove the arrows of the graph and have just simple lines?

Answer 1

If you already have a graph (that you could have imported via another method that doesn't support directed=FALSE or that you want to be directed but you just don't want to _ plot_ the arrows) you can also do:

    g<- df %>% 
      gather(key = "From", "To", na.rm = TRUE) %>% 
      distinct(From,To) %>% 
      graph_from_data_frame()
    E(g)$arrow.mode="-" #or whatever symbol that is not in c("<", "<-",">",">",“<>”,“<->)
    plot(g)

As stated in the documentation this allows to create directed graph with mixed style edges representation.