Is x plotted against y or is y plotted against x?

So, for simpler rule of thumb, I use: Wall against ground
So, whatever is your independent quantity, is usually plotted on the horizontal axis, as if it needs no support, like time. And the other quantity that needs support is plotted on (parallel) to the vertical axis. But of course, thats just to simplify things. In cases like Voltage and Current, we can interchangeably plot them, as V=IR, since both can be related to depend on each other, depending on what relationship you are trying to deduce. But to finally answer your question, the more common way to do so would be:

Vertical axis (y) against / versus Horizontal axis (x).

Usually, the standard graph is a plot of the dependent variable against the independent variable

(from a 2007 post archived in the Wayback Machine from a physics society forum hosted at the National University of Singapore)

The independent variable is normally plotted along the horizontal axis.

However, though in physical relations there may often be an obvious candidate for the independent variable, in pure mathematical equations, it is a matter of choice - often convenience - which one is chosen as subject of the formula. Also, there may be a reason to break with convention and swap the x-axis and y-axis over.

When describing a relationship that is represented on a cartesian graph, the correct form is

dependent variable against the independent (or manipulated) variable.

This reflect the fact that the value of the dependent variable is responding to, or following, changes in the independent variable. While it is common in classroom conversation to use the Latin versus rather than the English against, this should be avoided in formal writing because for many readers it carries a connotation of opposition that is inappropriate.

Your question embodies two separate mathematical conventions. The independent variable should always be plotted on the horizontal axis and it MAY be represented by the label x if there is not another more evocative label (such as t for time or c for concentration).

An example drawn from Chemistry might be

Figure 2.3 plots the rate of reaction r (in mL of hydrogen per minute) against the concentration of acid c (in moles per litre).