When should the label of the vertical axis be y, and when should it be f(x)?

When drawing multiple curves on the same Cartesian plane because they represent functions $f(t),g(t),h(t),$ etc. that share an input/independent variable $t,$ the vertical axis can be labelled the dependent variable $y,$ and the curves separately labelled $“y=f(t)”, “y=g(t)”, “y=h(t)”,$ etc.

When there is just a single function $h(t),$ the vertical axis can alternatively be labelled $h(t)$ so that the curve doesn't need to be labelled.

In either case, the horizontal axis is labelled the input/independent variable $t.$

It is okay to label the vertical axis as $y$ if the horizontal axis is labeled $x$ and $y = f(x)$. To compare two functions, say $f(x)$ and $g(x)$, both can be plotted in the same graph and labeling those functions through annotations or legends.

Note that $x$ and $y$ are just dummy variables. They can be replaced with anything, like $f = g(h)$, or $x = X(y)$. This means that you can compare $\sin\theta$ and $\cos\theta$ like the way you would in the previous paragraph.

However, to compare $f(a)$ and $g(b)$ would need additional information. Assuming that $f$ and $g$ are functions of $x$, comparing them would be similar to the first paragraph, just that we are tracking specific values instead of the entire function.