@mforns as per our conversation on IRC in late December of 2019:

Here is the gist: associating monthly data with the first day of that month is a bit confusing. For example, in the time-series graph here https://wmcs-edits.wmflabs.org/#wmcs-edits/wmcs-edits-timeseries T226663, see the results corresponding to 2019-12-01. Though these results are for the entire month of December, the way they are attached to 2019-12-01 implies it is either one-day data or data from the previous month. A small edition in ReportUpdater (RU) so that we don't have to provide the day of the month to RU and just the month and display data corresponding to a month on Dashiki would be a great achievement :)