On a position-time graph, why does the x-axis only show positive values?

Responses

The x-axis is a dependent variable, and a dependent variable cannot be assigned a negative value on a position-time graph.
The x-axis is a dependent variable, and a dependent variable cannot be assigned a negative value on a position-time graph. - no response given

The x-axis shows position, and position cannot be assigned a negative value on a position-time graph.
The x-axis shows position, and position cannot be assigned a negative value on a position-time graph. - no response given

The x-axis shows time, and time cannot be assigned a negative value on a position-time graph.
The x-axis shows time, and time cannot be assigned a negative value on a position-time graph. - no response given

The x-axis shows the motion of an object, and motion cannot be assigned a negative value on a position-time graph.

The x-axis shows the motion of an object, and motion cannot be assigned a negative value on a position-time graph. - no response given