Add the possibility to define stacked items in graphs from type normal.
Stacked items should stack on their next item below in the list of all items.

Example:
Memory usage graph with an items in correct order (top down):
1. "Memory used" (reporting 512MB, type stacked)
2. "Total memory installed" (reporting 4GB, type normal)
3. "Memory free" (reporting 1GB, type stacked)
4. "Memory buffers" (reporting 512MB, type stacked)
5. "Memory cached" (reporting 512MB, type normal)