How do I calculate values from a chart?

Newbie here! So please be kind if I am posting in the wrong forum!

I have a series of numbers (X and Y) from which I create a chart from as shown on the attached Excel spreadsheet. The chart is the smooth line type available in Excel.

The question is, how do I find the Y value corresponding to a given X? Basically if X is given I need to be able to calculate the corresponding Y.

Thank you all for your HELP! I greatly appreciate any assistance as I am currently stuck in what I am doing and need to get this solved!

Originally Posted by brooklynite

Newbie here! So please be kind if I am posting in the wrong forum!

I have a series of numbers (X and Y) from which I create a chart from as shown on the attached Excel spreadsheet. The chart is the smooth line type available in Excel.

The question is, how do I find the Y value corresponding to a given X? Basically if X is given I need to be able to calculate the corresponding Y.

Thank you all for your HELP! I greatly appreciate any assistance as I am currently stuck in what I am doing and need to get this solved!

Your file attached below.

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Originally Posted by yurttas

Your file attached below.

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Thank you yurtass for the file. The formula you created seems to be a "linear interpolation". Unfortunately in most of my data samples (I am trying to find one now) a linear interpolation will result in numbers that are way off. Can this be done using the same method that Excel uses to create a non-linear curve just like the one shown in my file? Especially that I will need numbers OUTSIDE the range too so I guess it would be EXTrapolation.

Originally Posted by pike

Its best fit

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It kind of works below 40 but above that it starts to distort a little bit.

Doesn't Excel actually give us the values it uses to produce the smooth chart? What is the formula that it uses?

Originally Posted by brooklynite

Thank you yurtass for the file. The formula you created seems to be a "linear interpolation". Unfortunately in most of my data samples (I am trying to find one now) a linear interpolation will result in numbers that are way off. Can this be done using the same method that Excel uses to create a non-linear curve just like the one shown in my file? Especially that I will need numbers OUTSIDE the range too so I guess it would be EXTrapolation.

Is this okey?


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Originally Posted by pike

Ok then you need to calculate m
with

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then to find b when x = 0
and use
y=mx+b for each line

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see attached work book

I like the slope idea and thank you so much for the info. The y=mx+b is if I assume LINEAR transition in between. Can you help me get a y=ax^2+bx+c or some other second degree formula? I think that would be a lot more complicated...