Support add=TRUE

[grantmcdermott]

Closes #55. Simplifies things by ignoring the legend for any added elements. (Maybe we can revisit down the line.)

Simple penguins example.

library(plot2)
par(pch = 15)

data(penguins, package = "palmerpenguins")

plot2(bill_depth_mm ~ bill_length_mm | species, penguins, pch = "by", grid = TRUE)

penguins$pred = predict(lm(bill_depth_mm ~ bill_length_mm * species, penguins), newdata = penguins)

plot2(pred ~ bill_length_mm | species, penguins, type = "l", add = TRUE)

^{Created on 2023-08-13 with reprex v2.0.2}