Interpolating an arbitrary function

Consider http://img521.imageshack.us/img521/369/chartov2.png

Function f(n,x) is numerically defined for specific values of x (the points) for specific values of n (the dotted lines connecting them).  Some of these values are coincident (on the chart, the right-most values of f(2) and f(3) in this case).  This is monotonic increasing with respect to both n and x.  It models a physical scenario, so parameters n and x are continuous, as is the range of f. 

Where n is one of the prior-known values and x is not, linear interpolation will suffice, similarly when x is prior-known and n is not.  But when both n and x are between their respective defined values, how to interpolate?

Ultimately, I am looking for a way to collate such a series of data, such that from the dataset I can calculate e.g. f(30,0.5).  No particular preference of language, an algorithm will do.

Forget it, it's NP-complete.

Uh uh uh! You didn't say the magic word.

LISP

>>1
By doing the same thing in three dimensions instead of two.

Have you read your NA today?

>>6
Yes, I am studying for my Numerical Analysis exam right now, in fact. Fuck Romberg.

>>4
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