Please consider an option for custom legends in graphs.

This option is in Cacti like for 10 years, it allows to add items to legend without drawing them to the graph and just simply print the item values. It's usable for any reference. 

Real life scenario is errors on wireless link (ber). It gives a cumulative number from SNMP, so in Zabbix it's only possible to print the value to graph (which does not make sense) or use simple change preprocessing function, but there is no way how to print the "Total errors" value in legend.

In attachment is like it works in Cacti. Total errors are printed as a label in legend and graph is empty, because they were no errors today.