Those are really calendar weeks (although starting on monday independently
from locale) as it is less expensive to compute them accurately for table
precision than for graph precision (much fewer dates to convert).

Default behaviour doesn't change.
If --fixed-yrange is provided with a negative value (default), ymax is
left floating.
If --fixed-yrange is provided with a value, ymax is fixed at that value.

This bug caused mismatch between links and sections.

Some are corrected, others are TODO.

To find which user agents should be ignored (bots, monitoring, ...).

Also, switch "week" definition (graph granularity for "--period quarter")
from "7 days chunks starting at 1st of month" to "7 days chunks starting
at 1st of january", so fewer dates need tweaking.

Similar to cb29ab44 .