LTSpice: When running a linear AC simulation, how to view the voltage ratio between two voltages?

As you can read here, LTSpice can plot a mathematical function of the traced variables. You just have to edit the plot to specify the functions.

An example from the from the linked page is shown below. As mentioned in the comments, to edit the plot function you can right click on the name. The plot name is the green string starting with "1.1*pow...".

What you see in an AC simulation is the ratio, but expressed in decibels. To see the ratio properly you would need to change the Y axis from logarithmic to linear. (right click)

This will be expressed in volts still but you can change the expression from V(y) to V(x)/V(y) as in crj11's answer.