x values for y input on graph

[idrisi]

hello,

can some one help me, i am trying to find the intercept points on a plotted graph so that if i input a y value i can get the corresponding x values of the plotted line which intersect at plotted line. please see attatched VI. 

to explain again what i am aiming to do is essentially (this may be another way to do it) is postion two cursors in the graph and get the x and y vaules although i would like to specify a y value.

also in the attached file i have set a property node which displays the x value for a y input but i cant do the same for both cursors, could i also get some help on that  

[TCPlomp]

here's a snippet showing you a possible solution:

It has one problem, if the Y-values are not increasing it won't find a correct match.

Ton