A parallel coordinates plot represents a 4D point $(x_1, x_2, x_3, x_4)$. If a line segment connects $x_1=2$ to $x_2=6$ and $x_2=6$ to $x_3=10$, what is the coordinate $x_2$ at the intersection of this line with a vertical axis placed halfway between $x_1$ and $x_2$?