Graph widget incorrectly shows the problem period

Steps to reproduce:

1. Create graph widget with enabled "Show problems" option
2. Generate problem for one of items from this widget and resolve it after some time
3. Open dashboard and select a period including the beginning and end of the problem
4. Result: the graph displayed correctly
5. Select a period including the beginning of the problem
6. Result: the graph displayed correctly
7. Select a period including the end of the problem
8. Result: the graph displayed with the incorrect problem area
9. Select a period including the end of the problem and the future
10. Result: the graph displayed with the incorrect problem area in the future