Already used color autocompleted in graph widget edit form

Steps to reproduce:

1. Create a new graph widget.
2. Delete pattern select data set (user wants to create graph with only item list)
3. Add item list dataset.
4. Add one item in item list dataset.
5. Save widget.
6. Open widget for edit.
7. Add one more item.

Result: Both items have same color.
Expected: Items should have different colors from our color palette.