Extend point_forecast option to plotting functions

[mitchelloharawild]

library(fpp3)
eggs <- as_tsibble(fma::eggs)
fit <- eggs %>% model(RW(log(value) ~ drift()))
fc <- fit %>% forecast(h=50)
autoplot(fc, eggs, point_forecast = c("mean", "median"))

Use the linetype guide to indicate the point_forecast method.