Hydrostatically insensitive testing and injection plug

A plug for use in a wellbore has a piston that is movable on a body. A control valve is disposed in communication between the wellbore and a piston chamber and can capture wellbore pressure in the piston chamber. A closure on the body can transition from a closed condition to an open condition relative to the port in response to the movement of the piston. A fixture releasably holds the closure in the closed condition on the body. The fixture releases in response an increased pressure differential on the piston above an initial pressure differential between the captured chamber pressure and the wellbore pressure.