Wants to calculate and show intersecting point on Graph

How much is a charting question and how much is a spreadsheet question? In order to show a vertical line connecting (x,0) with (x,f(x)), one first needs to calculate y=f(x) in the spreadsheet (the chart cannot do this for us).

If this is exclusively a charting question and you have already calculated y=f(x) (maybe this is the value F25??), then adding the lines to the chart should be as easy as adding another data series. In a convenient place, enter the data for the new series. Perhaps in E27 enter =$E$25, copy into E28, and enter 0 into E29. In F27, enter 0, In F28 enter =$F$25 and copy ing F29. Then add a new data series using E27:E29 as the X values and F27:F29 as the Y values.

If the larger question is how to calculate f(x), I'm not sure how much we can help without knowing something about the industry you are working in and what are considered the best practices for this sort of thing. In a generic sense, there are usually two strategies for this kind of thing -- regression and interpolation. Since I doubt any of us knows the best practices for your industry, I don't think any of us can decide on the best strategy for this. If you need help deciding how to calculate f(x), we need you to help us understand the process/algorithm for performing the calculation. Then we should be able to help you program that calculation into the spreadsheet.