The code below generates the graph I want to have. Except that I want to add values l to the tooltip. Currently I have no idea how to do this.
require(googleVis)
df <- data.frame(x = c(1:5), y = c(10, 20 , 30, 40, 50),
l = c("A", "B", "C", "D", "E"))
plot(gvisLineChart(df, xvar = "x", yvar = "y"))