Tracking Data usage is only available for myPlan Monthly Plans.
To check this out, log in to My Account here. Don't forget to register for My Account if you haven't done it yet. Then pick View My Bill. On the bottom left hand side you'll see your unbilled usage details and unbilled usage summary. Unbilled usage details show you the type of usage, when and where it took place, and the length of time spent doing each thing. You can also check your usage right from your Virgin Mobile phone with the My Account app.