Allow to add vertical lines for developmental stages

[timflutre]

Description

After a call to get_sim and to get_dev_stages for four stages, I plot the sim as curves through time (plot(sim)) followed by custom code to add four dev stages as vertical lines on the same plots:

sim <- get(sim(workspace, usm)
dev_stages <- get_dev_stages(sim, c("iamfs","ilaxs","idrps","inous"), usm)
p <- plot(sim)[[1]] + geom_vline(data=dev_stages[[usm]], aes(xintercept=Date, linetype=var)) +
      scale_linetype_manual(values = c("dotted","longdash","dashed","dotdash"))
print(p)

However, this adds four extra "facet" plots, one per dev stages.

Expected change

I would like a code as above (or similar) so that the dev stages appear as vertical lines on the same plots as plot(sim), not on extra facet plots.