I can't reproduce your issue....
library(tidyverse)
set.seed(42)
ex_df <- data.frame(
Date=seq.Date(from=as.Date('2019-01-01'),by="day",length.out = 365),
Depth =30+cumsum(sample(c(-10,10), size=365, replace=TRUE)),
Type = factor(sample(c("Natural","Artificial"), size=365, replace=TRUE))
)
plot <- ggplot(data =ex_df, aes(x = Date, y = Depth, colour = Type)) +
geom_point() +
ylim(0, 200) +
theme_minimal(base_size = 10) +
geom_smooth(color = 'Dodgerblue', method = 'gam', se = FALSE) +
scale_colour_manual(name = "Type of Snow", values=c("#99d8ff", "#2e4d82")) +
geom_hline(aes(yintercept= 100, linetype = "Current Year Mean"), colour= 'green') +
geom_hline(aes(yintercept= 80, linetype = "2010 Mean"), colour= 'black') +
scale_linetype_manual(name ="", values = c('dotted','dotted'))
plot