This function returns a plot of the vaccine coverage or the
cumulative coverage (if cumulative = TRUE). The return is a 2-axis ggplot2
element with the number of vaccines per date on the left axis and the
coverage per date on the right axis. When a matching routine is performed,
the left axis also accounts for the doses of the matched cohort.
Usage
# S3 method for class 'vaccineff_data'
plot(x, date_interval = NULL, cumulative = FALSE, ...)