If you already have the Week field in your QVW, you just need to create a cyclic group with Month first and than Week. And then delete the Month dimension of the chart and after add the cyclic group as dimension.
If you don't have the field Week or if isn't this the problem, please attach an example application (QVW).
For your solution to work,the user has to click on the cyclic icon to switch between the months and weeks. is there a way to automatically change the axis values from months to weeks based on the selections? Assume that the user select a month or two, then the chart axis value should switch to weeks instead of months. is it possible?
The Drill-down Group is the way to automatically change the axis values from months to weeks based on the selections, but to perform the drill-down you have to select just one month. The group icon (button) shold be used to drill-up.
If you don't want to use groups, you can try to create 2 charts over positioned: one with month as dimension and other with week as dimension. Then use the Conditional Show property (in Layout tab) to show the Week chart only when there is at least one month selected (for this, use GetSelectedCount () function). If you clear the selection, the Month chart will be automaticaly showed.
Drill down group will be the best option for you. Just create a new group on the dimensions tab and choose the radio button for drill down and add in both your month followed by week fields. Then use this group as your chart dimension.