Do you have one of these?

My husband and I are empty nesters and we're about to move from our current home.
We are trying to get it in tip top shape to sell, so that would include the ususal:
cleaning, removing clutter, and painting.
Without thinking, I almost wipe this clean, thinking that it's just a dirty spot. 

Do you know what it is?

If you guessed some type of measurement, you'd be correct.
It's a record of our boy's height as they were growing up.

It was a ritual.

The boys would walk by and say, "Dad, you better measure me, I think I grew!"

How can I Windex this all away?

I'm thinking we need to take it with us...replacing a door frame isn't too hard, right?