What does the axis of this diagram represent?

The house quaked in the earthquake.

My father works at a famous think tank.

He was attacked personally.

Human trafficking is prohibited all over the world.

My grandfather was diagnosed with shingles last week.

My grandfather got shingles last week.

He was hit hard in the solar plexus during the boxing match.

Neuroglia play an important supporting role in the nervous system.

I love synthpop music.