How do I define a derivative of a function and then immediately use that?

Question

In other words, how do I concisely express something like "derivative of f(x) with respect to x, evaluated at t"? As in:

wxplot2d(
    [
        parametric,
        realpart(zeta(1/2+t*%i) / <derivative of zeta(z) with respect to z evaluated at 1/2 + t*%i>),
        imagpart(zeta(1/2+t*%i) / <derivative of zeta(z) with respect to z evaluated at 1/2 + t*%i>),
        [t,14,80]
    ],
    [same_xy]
)

What is the syntax I should use instead of the bits in angle brackets?