I would really like to see the ability to use a custom defined range, which is not a field from my data source, as an axis. This could then be referenced with calculated fields to do analysis which is currently very difficult in Tableau. For example, I would like to plot the number of live cases over time, calculated at weekly intervals. So what I want is a custom range as my axis which is each Monday from say 2012. I then create a calculated field that sums the number of cases where Date Opened < Axis Date & Date Closed > Axis Date or Date Closed is null. This currently only seems to be possible by doing a lot of the work at the sql level, and creates an unnecessarily large data set in the process.
All the tools to define such a custom range exist already in the form of parameters. If I drag a parameter to an axis though it just displays the current value. What I want is to be able to have all possible values for the paramater displayed along the axis. I think this would be enormously useful and not especially difficult to implement.