Surely you know that if P= @r then @= P/r?
If you know that, at least approximately, P= @r, and want to determine @, choose any (r, P) pair and divide: @= P/r.
If P= @r exactly then it does not matter which point you use, you will get the same value for @.
If it is not exact, the value for @ will depend upon which point you choose but if "approximately", P= @r, the different values for @ will be "approximately" the same.
(Note that "P= @r" includes the point r= 0, P= 0 for any @. If r= 0, P= 0 is not given but the graph still looks like a straight line, use the more general "P= @r+ b", a straight line that, for [tex]b\ne 0[/b] does NOT go through the origin. In that case, you will need to use two points to get two equations to solve for @ and b. Geometrically, two points determine a straight line.)