Data Feeds started showing up as "Null"


All my data feeds started showing up with a lot of “nulls” starting in late September. I am attaching a sample graph for my home power to illustrate the effect. If I go into the dataset, I do see sporadic valid samples, but mostly just “nulls”.

Thanks for any guidance!

Where does the data originate?
How does the data travel to emoncms?
What type of feed is it?
How often does the input get updated?
What version of emoncms?
Is this a low-write emoncms (eg emonSD/Pi)

Thank you for the response. I realized that the problem had to be in the input to feed mapping: when I looked at the input page, it always showed the correct values. I just deleted my old data and recreated the feeds as timeseries (it used to be 10 sec interval). Things are working well now, but I am still unsure why things crapped out after a year.

The data source is a Vera Home automation unit. I have it upload data whenever a value changes by a certain amount, and also every 20 minutes.