What would a negative externality diagram look like if the initial price is equal to 0 and then increased with tax? The price of plastic bags in Scotland has been increased from free to 0.05 pounds sterling. How would I represent the negative externality decrease using a diagram in such a situation?

This is what I have so far. I'm not sure that it is correct.
Prior to the tax, the marginal private cost is zero (so the MPC curve is a horizontal line along the x axis).

The individual chooses to consume where marginal private benefit is equal to marginal private cost (i.e., where the MPB curve crosses the x axis).

A tax of 0.05 implies that the marginal private cost (i.e. the cost per unit consumed) is now 0.05. This means that we get a new MPC curve that is a horizontal line at p=0.05. The new privately optimal consumption occurs where this new MPC curve intersects the MPB curve, which results in a lower quantity consumed.