bar report element autocaptions generated once only

in bar reports add some element with automatic caption, for example, time period in "distribution of values for multiple items".
edit this time period, change start or end date. notice how label has now been set to manual and is not changed - user has to notice this and uncheck the checkbox.
instead, checkbox should always stay unchecked unless user has checked it manually.
if it's cheaper to pre-generate captions, maybe that requires additional field in the database or something...