[SOLVED] Determine equation based on dataset

I am currently working on something for my personal site. Most things I can figure out myself, but I'm having trouble with this problem.

I need to determine an equation that's based on the following dataset:

x=1/4

y=4

x=sqrt(2)/4

y=2

x=1/2

y=1

x=sqrt(2)/2

y=1/2

x=1

y=1/4

Alternatively, it can be expressed as (in radians):

x=sin(PI/6)/2

y=4

x=sin(PI/4)/2

y=2

x=sin(PI/6)

y=1

x=sin(PI/4)

y=1/2

x=sin(PI/2)

y=1/4

Now what I'm trying to do is find the equation these numbers represent.

Let's say I know y=3. How can I determine x based on the information above? I see a pattern, but I'm unable to come up with a working equation.

I am able to test to see if a pair of values is correct.