Help locating x with given y in a line graph

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For example I have a set of x and y coordinates like this

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I put them into a line graph.
Then what I want to be able to do is pick lets say 2.25 on the x axis and have it return a value for y.
Any help on this matter will be greatly appreciated. Thanks in advance Andy.

[Andy Pope]

See attached.

I have used some formula to interpolate the Y value from the data table.