Ever since I first saw this xkcd cartoon, I wanted to use it in a lesson. I finally put that together this year. I used the cartoon as a way to help convey the idea that points on a coordinate plane are a way to easily visualize the relationship between two different variables. The purpose of the numbers is simply to quantify those relationships, if such a quantification is necessary. I then had students make their own graphs for homework, with variables of their choice. If I had more time to spare, it would have been nice to do this in class (and the outcome would have been better, I think).
This lesson (Unit 4, Lesson 5) and others can be found in my box.com widget to the left. I recently updated Algebra 1, Units 3 and 4.
Here are some examples of the students' work.