ssrs 2008 - How to setup custom number format for Year and isoweeknumber like "YY'WW" -

i have below problem. putting below function in custom number format x axis of chart. works years , mohths when choose weeks, changing date yy'ww (year'week) format.

can please me on this, if have idea? need change in parameters!vtimescaleunit.value="weeks","yy'ww" code.

=switch(parameters!vtimescaleunit.value="years","yyyy",parameters!vtimescaleunit.value="months","mm"&"/"&"yy",parameters!vtimescaleunit.value="weeks","yy'ww")