An alternate approach to Treeview

I have a screen where I get a List of Main items. Inturn, each of these main items will have list of sub items inside them.

Now user can select a Main Item or items inside it and then run the report. This will generate a chart for each item selected (in our case 7 lines on chart).In my view this is not a good user experience as the list of sub items can grow really long and user has to scroll through.

Please not there is no relationship between the main item selection and child items. When we select Main item,it does not select all the child items beneath it.

An alternate , I could think of is having a side tab for each Main item and the show the Sub items on right side.