There is a sine and cosine function but no tangent function. Anyone have a function of their own?

The simplest way would be to use the trigonometric identities.

Specifically, the one that says tan θ = sin θ / cos θ.

That gives:

function tan(a)
	return sin(a)/cos(a)
end

Note though that this starts to fall apart in the vicinity of 90° and 270°, since pico-8's fixed-point variables can't represent the large numbers around those values, or the infinite values at them, but realistically you wouldn't be using tan() for angles like that anyway, because it wouldn't be useful.

@zep

This is another one of those functions that I think it's dumb for pico-8 not to include. Yeah, we can make it ourselves, but... seriously... just include it, it's a fundamental thing.

@Felice

Thank you!