Previous Zabbix versions displayed an indicator in the lower right corner of graphs, letting the user know whether data comes from history or trends.
This was useful to know what granularity the viewer may expect.

Around Zabbix 3.4, this was intentionally removed (only available when debug mode was enabled, not something to be done for normal users).
The stated reasons changed, starting with "it takes away space on the graph", ending with "it clutters display". The first one does not seem truthful, as this indicator did not take any space (compare the attached examples).
The second does not seem truthful either, as many users had not even noticed this indicator (but were happy to find out it is there).

Please consider returning this useful indicator.