Add legend to a plot with R

legend( "topleft", c("cars", "trucks", "cars1", "trucks1"), 
text.col=c("blue", "red", "yellow", "green") )

Use the ?legend command to see options, such other locations on the graph, besides "topleft" or text size, whether to have a box around the legend, etc.

For example

legend(
  "topright", 
  lty=c(1,2,1,2), 
  col=c("blue", "red", "yellow", "green"), 
  legend = c("cars", "trucks", "cars1", "trucks1")
)

See ?legend.