Explanation by Jonathan Drummey:
- The size of the Gantt bars is such that multiple weeks are crossed for a
given mark (Gantt bar).
- You want to enable the user to have a "filter" where they can show a
given range and make sure that any bars that overlap into or out of that
range are displayed.
- If you use WEEK(Date) as a Quick Filter on an unpadded data source, as
soon as you filter out a week that is the start date of a mark, Tableau
removes the entire mark from the view. This is Tableau's standard behavior,
when we use a Filter on a dimension we are telling Tableau to only return
the desired data from the database. In SQL terms, the filter is added to
the WHERE clause of the query that Tableau makes to the data source.
- If you manually fix the axis range, Tableau will only display the marks
(or parts of marks) for that axis range. Though this can have the same
visual effect, this is not a filter in Tableau's terminology. Tableau is
still making the same underlying query to the data source, and still
computing the same number of marks, it's only displaying the ones in the
The padding solution you came up with is what
I'd use as well. There are other solutions for padding using table calcs,
however then you'd lose that in-place filtering effect that Tableau
supports when a dimension is being filtered.