The data that is uploaded to the website for the current month consists of "raw" but edited 15-minute data.
Every night at between 00:30 and 01:00 every gas analyser carries out an automatic calibration. This calibration data is removed automatically from the data uploaded to the website (but held within the databases for QA/QC purposes) and replaced with a zero for each 15-minute data point. It is these zero data points that are plotted on the 24-hour daily chart. The daily chart is comprised wholly from 15-minute data.
Please be aware that if the chart time-frame extends over two or more days, the data presented within the chart is in the form of hourly averages and the calibration data points are not therefore easily seen.