auto: Determine colours automatically. If color is already specified in
milestones tibble, this will be used. Otherwise, the colour scheme is determined by
milestone_palette_list$auto.
given: The milestones object already contains a column color.
cubeHelix: Use the rje::cubeHelix() palette.
Set3: Use the RColorBrewer::brewer.pal(name = "Set3") palette.
rainbow: Use the grDevices::rainbow() palette.
milestones
Tibble containing the column milestone_id (character).
If color_milestones is set to "given", this tibble should also contain a column
color (character), containing colour hex codes (e.g. "#123456").
layout
The type of layout to create. See ggraph::ggraph() for more info.
arrow
The type and size of arrow in case of directed trajectories. Set to NULL to remove arrow altogether.