How to create scripted dataset

Could someone help me in scripting to create a dataset. I am using WorkItem_Snapshot as snapshot and Workitem_states as table. My requirement is to get total no. of workitem opened each week and also based on severity.  So I need to draw line graph of open work item trend chart. Workitem_states gives workitems opened each day with severity and WI_Count. As I am looking for split up based on severity, I am not getting the total count of work items. So I wanted to have another dataset with one row for each date, then different severity of work items opened on that date and total of work items opened. To achieve this how the coding to be done?