If someone shuts off the rollup.py, that will definitely break the pggraph instance.
You will continue to gather data, but the display of the data will be affected, because the raw data will not be manipulated and loaded into the appropriate tables for display.
To fix this, you need to run the history_rollup.py, which is under /grapher/history_rollup.py in the repository/checkout.
This will truncate the current history tables, and run the rollup from the beginning of time for your particular instance.
After which, any missing display data will be displayed on the graphs.
The script can take a long time to run, if the data gathering has gone on for several months or more.