palette("Tableau 10")
library(plot2)
# Density plot with manual colour and fill (via the 'bg' arg) 
with(
  airquality,
  plot2(
    density(Temp),
    by = Month,
    col = "white",
    bg = "red",
    grid = TRUE,
    legend = FALSE
  )
)

# Density plot with automatic, grouped colour and fill (via bg = "by")
with(
  airquality,
  plot2(
    density(Temp),
    by = Month,
    bg = "by", # or: type = "ribbon"
    grid = TRUE
  )
)

# Note that the "filled" legend is adaptive for other types of ribbon plots.
# E.g. Here we don't get a border, but rather a line in the middle.
mod = lm(Temp ~ 0 + factor(Month) / Day, airquality)
pred = predict(mod, interval = "confidence")
with(
  cbind(airquality, pred),
  plot2(
    x = Day, y = fit, by = Month,
    ymin = lwr, ymax = upr,
    type = "ribbon",
    grid = TRUE
  )
)