Managing fractions of time units in gantt charts

[dylms]

gantt.py works by cases (= time unit) and can't manage fractions of cases.
Hence on weekly/monthly scale, tasks that starts or ends during a given week/month takes all time unit on the chart. Visually, we're tempted to understand that the task is due on the entire week/month (e.g. snapshot), or that 2 tasks on the same unit without overlapping period are processed in parallel.

It would be nice to manage time unit fractions in order to avoid these misinterpretations.