The graph shows three parallel time series. One tracks the reported daily number of death certificates during 2020, colored red. Another represents the aggregate average number of daily death certificates between 2017, 2018 and 2019, in gray. A final series adds to the one described above, the daily number of COVID-19 deaths reported for that same day in CDMX.
Red: Daily death certificates issued for 2020 | Gray: Average 2017--2019 | Yellow: Average 2017--2019 + COVID-19 deaths reported in CDMX
To see a more detailed look at the building process, you can read my post in Medium.