Simple Gantt chart plotting function.

gantt_chart(slots, labels, active_color, passive_color)

Arguments:

slots: Matrix with rows per activity.
- Columns should correspond to alternating begin and end times (absolute) of an activity.
- Rows should be padded to the right with zeros if number of slots is unequal for different activities.
labels: Column containing activity labels.
active_color: Color of active slots of an activity.
passive_color: Color of passive slots of an activity.