Expression apply on Y value only

I have added an expression to a puppet pin so that this pin is parented with a Null. But is there a way to only parent the Y value with the Null?

I'm very bad with expressions so I don't know how to explain, but let's say..

Both X and Y values are red now, meaning they're both animated. But I want the X value to be locked, so that it won't be animated.