Modifying a `gg_vistime` using a `scale_color` function changes text color and adds legend

[jabenninghoff]

Modifying a gg_vistime object with a ggplot2::scale_color function unexpectedly changes the text color and adds a legend.

Reproducible example below. This chart should look like the vistime example but does not.

library(ggplot2)
library(vistime)

timeline_data <- data.frame(event = c("Event 1", "Event 2"),
                            start = c("2020-06-06", "2020-10-01"), 
                            end   = c("2020-10-01", "2020-12-31"),
                            group = "My Events")

gg_vistime(timeline_data) + scale_color_discrete()