Determining x values from y values from a curve

If I have the following data: 0 to 5 in increments of 0.5 for my x values and my y values are 4.9, 4.9, 4.6, 4.4, 4, 3.2, 1.2, 0.5, 0.2, 0. This set of data forms a curve. If I want to find the midpoint y being 4.9/2 = 2.45, how do I find the corresponding x value? And can you insert a line of some sort to show this point with respect to both axes?

If I want to find the midpoint y being 4.9/2 = 2.45, how do I find the corresponding x value?

2 basic approaches that can be used: fit the entire data set to a curve (probably a logistics type curve for something like this) or use linear interpolation. Linear interpolation is probably easier, and has been discussed many times. In post 18 of this thread (http://www.excelforum.com/excel-gene...of-curves.html) user shg has a sample spreadsheet with a UDF that may be useful for this purpose.

And can you insert a line of some sort to show this point with respect to both axes?

I think the easiest way to do this will be to add another series consisting of that point, then format it with appropriate error bars. http://office.microsoft.com/en-us/ex...310.aspx?CTT=1
http://office.microsoft.com/en-us/ex...044.aspx?CTT=1

You have to do this with interpolation. There are two approaches to this.

The simple approach is to take the two y values that bracket your target value, and do linear interpolation between those two points. In your case you have the two points (3.0, 3.2) and (3.5, 1.2). To interpolate 2.45, the answer is

3.0 + (3.5-3.0)*(3.2-2.45)/(3.2-1.2)

= 3.1875


The difficult but more precise approach is to model your curve. For example, your data does not work well as a linear model. It looks like a third degree polynomial to me, and when I add a trendline Excel gives this equation:

y = 0.1134x^3 - 1.0583x^2 + 1.4868x + 4.6252

You want to start with y, and derive x. That's harder. That means you have to solve this equation for x and I don't know how to do that for anything higher than a quadratic. Someone else probably does.

On review, I see MrShorty has beaten me with basically the same ideas.