For my understanding, using /feed/data.json with mode is only relevant for cumulative feeds like total kWh or total gas used. This is used by the bargraph and other apps to display total per day/week/month.
When the feed have no missing values (no nulls) at midnight (in the user time zone), this works fine.
But if the input was not sending data during midnight, the feed will have null and the result for that day will be null as well, causing the graph to show nothing at that day even if the feed data 10 seconds before have value.
Since the API only needs to support cumulative feeds (and only the PHPFINA and PHPTIMESERIES engines), I think the behaviour should be to look back to the latest existing value.
That is: starting at the midnight position, if the value is null - get the value of the previous position (but only go back until the position of the previous result point or no more than a day?).
The point is: if the data at midnight is missing, it is better to return the data that we have just before midnight to calculate the total for the day/week/month than to show nothing at all.