Typical - I've been trying to work it out for a day, then I post and immediately find out how to do it.
For future reference, I selected the Calendar Date axis and changed it from 'Automatic' to 'Independant axis ranges for each row or column' - seems easy when you know how!
Glad you answered your own question. When I was a programmer we had a full-size cardboard cutout of a person. Anytime you'd been trying for hours to fix one niggly bug, we would explain the problem to the cardboard cutout - 90% of the time, the very act of explaining the problem presented the solution.