group by two factors

dd <- mtcars %>% data.table()
col <- c("gear","carb")
dd[ , (col):= lapply(.SD, as.factor), .SDcols=col]

p <- dd %>%  ggplot(aes(x=hp, y=disp, color=carb)) +
\t\t geom_point(aes(shape=gear))  
p <- p + geom_line(aes(group=interaction(gear,carb)))
p