See to the attached picture.

Without the text "(last value)" it always a unexpected news that exactly last value will be used, even for developers.
Last value it's not last value visible on graph for selected period of time, but the latest value stored in the DB "items" table.
This text even could be "(latest value)".

The text "(for trends)" will help to understand that this option affects only when selected period of time is enough big (~3days and more) and when data will be shown from trends.