Scatter graph problem

[jmturrell]

Right im new to excel so please bare with me
Ive made a scatter graph in excel 07. concentration (x axis) vs absorbance value ( y axis)

ive put a poly trendline on it and basically i want to know how to find the x value if ive only got the y values. so using the trendline i match it up to work out what the concentration of each value is.

i would post attachment of my graph but can't!

i have been told to use the goal seek tool... does anyone know how i can do this??

please help
Thanks

 

[FrigateSkimmer]

jmturrell,

"ive put a poly trendline on it and basically i want to know how to find the x value if ive only got the y values. so using the trendline i match it up to work out what the concentration of each value is.

i would post attachment of my graph but can't!"

What kind of shape has the polynomial given you - a simple curve or a complex curve (i.e. are there unique values of x for any given y)?

It sounds as if you have the independent variable (cause) on your y-axis. If my parenthetical condition exists, then you might try graphing the data by transposing the axes - swap x (effect) for y (cause). Then, you can use the formula of the trendline to fill in the gaps within your data interval.

[Note: be sure that when you use the EXCEL installed trendline, you select the formula when it's presented, right click for Format, and choose the Number tab to give the coefficients the same precision found in your data. Otherwise, you may be using rounded coefficients, and the trendline will be incorrect for your data.]

FS