Word to describe something on the X axis

Consider the following chart.

The word "series" is used to describe something unique about the data such as the city.

There is also something common to each series curve such as the month represented on the X axis. While this example used time as the basis of the X axis, another chart could have used something else such as elevation.

I am designing an application were the user can add a new series as well as a new X axis point. I am thinking of two buttons Add New Series and Add New X Axis.

I feel the word "series" works (correct me if you feel differently), however, I don't know a good word to describe the x axis.

What word should be used?