One or more meters that gather data for this graph are offline. You can check the app for the last known Wi-Fi signal strength percentage at Settings > Meters > 'Your offline meter' > Wi-Fi > ''name'' %. If the signal strength is below 50% you might experience temporal offline statesIf the signal strength is below 20% your device will be offline often.
If you are having signal problems you might try to reposition your Wi-Fi router for better coverage. This can be done by rotating your Wi-Fi router, or repositioning your router.