Determining values at an arbitrary position

Determining values at an arbitrary position

Given is the following image:

I want to determine D, E and F in the aforementioned drawing. D, E and F
are equidistant; A, B and C are not.
A, B and C (and respective crosses in the chart) are given values. Using
A, B and C(and more values, this is just an example), what could a
function look like I can give E(for instance) as a parameter and that
outputs a new Object containing the current value and all Strings(A, B...
are strings aswell as values) that occured within.
Other example: Let's say F equals 100.
I run a function with f(100,50), meaning current value at 100 is desired,
distance between values is 50. I want to get an object with say 20 for the
value aswell as the strings B and C as they occurred between E and F(which
have a difference of 50).
If no event occurred between the desired value (at 100) and the previous
(at 50), output null.