This will occur if the scenario that you ran had a VDDT Output property defined but had the save output to the database checkbox deselected. In this case no output is saved to the database so it is not possible for Path to generate graphs of results.
The VDDT Output property can be found in the Advanced properties tab of the scenario properties under VDDT Settings in the property tree.
You will need to select this checkbox and rerun your scenario to be able to graph results.