I just rewrote gnumeric_interpolation to use collect_float_pairs. In testing the new implementation I noticed the following issue which I also confirmed in the previous implementation (ie. I didn't just introduce it):

------------------------------------------
#1:

A1:A3  1,2,3
B1:B3  1,2,3
C2:C4  1,2,3

in E1:E2 enter the array function:
=interpolation(A1:A3,B1:B3,C1:C4)

I would expect to see the result:
#VALUE
1
2
3
but indeed observe
#VALUE
0
1
2

This is clearly not correct.

------------------------------------------
#2

A1:A3  1,2,3
B1:B3  1,2,3
G1:G4  1,2,,3 (ie, G3 remains empty)

in H1:H4 enter the array formula
=interpolation(A1:A3,B1:B3,G1:G4)

I would expect to see the result
1
2
#VALUE!
3

but indeed observe
1
2
#VALUE!
0

This is clearly not correct.