Chart date labels don't update (using dynamic named range)

Okay, sometimes Excel 2010 really drives me nuts and today is sadly one of those days :(

Anyway... I'm trying to make a chart with selection 2 options to it: 1) startdate 2) number of months.

All (seems to) works well in terms of the dynamic named ranges and in the chart HOWEVER the labels on the horizontal axis won't update along with the rest as soon as the startdate droplist has been changed to another selection.

I suspect that it's somehow a date formatting issue but I can't figure out what's wrong where.

Anyone any idea?

See attached document for the issue.

Cheers,

Steven