如何为每个多图添加标签？

Question

p = ggplot() + 
  geom_line(data = Month_time, aes(x = Month, y = CarrierDelay), color = "red") +
  geom_line(data = Month_time, aes(x = Month, y = WeatherDelay), color = "purple") +
  geom_line(data = Month_time, aes(x = Month, y = NASDelay), color = "yellow") +
  geom_line(data = Month_time, aes(x = Month, y = SecurityDelay), color = "green") +
  geom_line(data = Month_time, aes(x = Month, y = LateAircraftDelay), color = "blue") +
  xlab('Month') +
  ylab('Delay Types [min]')
print(p)

如何为每个多图添加标签或图例？