ggplot(fortify(n, arrow.gap = 0), aes(x, y, xend = xend, yend = yend)) +
  geom_edges(color = "white", alpha = 0.5) +
  geom_nodes(aes(color = period, size = log10(degree))) +
  guides(alpha = FALSE, size = FALSE) +
  scale_color_manual(values = wes_palette("Zissou")[ c(1, 3, 4, 5)]) +
  theme_blank(14) +
  theme(rect = element_rect(fill = "grey10"),
        panel.background = element_rect(fill = "grey10"),
        text = element_text(color = "white"),
        legend.position = "bottom")