To create a user-defined slice for cost plan data, you can accomplish in using the following steps:
1. Go to Administration, Time Slices, click 'New' Button
2. Create the new definition and pick the appropriate Investment type and Planning data
Example: I have created a Monthly Slice for the total planned cost for projects ; there is also separate items for capital and operating which can be used as well.
Try this out and query the slice ID that is created and see if you get the data that you need for your reports.