If you only want this to affect a single chart, you probably don't want to make any selections. Instead, you'll probably want set analysis. As an example, say your chart has an expression of sum(Sales). To restrict this to only the current year based on your variable, you might use an expression like this.
sum({<Year*={'$(_SySCurrentYear)'}>} Sales)
The exact expression may depend on how your field and variable are defined, though.
