The heatmap shows the activity as reported by the RBN (CW/PSK/RTTY, no FT8) of the last 12 months. You can scroll back to January 2015.
Daily activity is measured by the number of active hour slots (24) times band slots (13, 160m - 2m incl. WARC and 60m) and the number of continents in which a skimmer spotted the station in the respective hour, for a maximum daily score of 24 * (13 + 6) = 456. When filtering by band, the continent information is omitted, because it's only saved once per hour slot, not per band.
Click a single day on the heatmap to see its details.
<a href="https://rbn.telegraphy.de/activity/OY1CT"><img src="https://rbn.telegraphy.de/activity/image/OY1CT"></a>