R language draws a simple partially directed connection graph

library(igraph)
g <- make_empty_graph() +
  vertices(letters[1:10]) +
  vertices("foo", "bar", "bar2", "foobar2")
g <- g + edge("a", "b", color="red")
g <- g + edges("foo", "bar", "bar2", "foobar2", color="red", weight=1:2)
plot(g)

Reference: "Network Analysis and Visualization" page 11