The list used to create the chart. Elements are:
labelsThe task labels to be displayed at the left of the chart.
starts,endsThe task starts/ends as POSIXct dates/times.
prioritiesTask priorities as integers in the range 1 to 10.
There can be less than 10 levels of priority, but if priorities do
not start at 1 (assumed to be the highest), the default priority colors
will be calculated from 1.