# Thread: XY Graph and curve fitting (2003)

1. ## XY Graph and curve fitting (2003)

Hi everyone!
I'd like to know which are the method applied by Microsoft Excel in the XY-Graph when we want to fit a curve (interpolate): linear, logarithmic, polynomial, exponential, etc. Does anyone know which is the method applied in each case?
Thank you in advance and I know it is hard to put up with my english. Sorry.
Bye!

Federico.

2. ## Re: XY Graph and curve fitting (2003)

For linear and polynomial trend lines, Excel uses the standard least-squares algorithm. For example, a line has equation

y = ax + b

where a is the slope and b is the intercept with the y axis. The vertical distances of the data points to the line are calculated; these distances are squared and summed. The program determines the values of a and b for which the sum is as small as possible. It is possible to solve this explicitly

For a higher degree polynomial it's similar, but with more coefficients, and in general, the solution is found by iterative approximation.

For logarithmic and exponential trend lines, Excel does the following:
- It calculates the exponential or logarithm of the y values.
- It uses the least-squares method described above to find the equation for the linear trend line through the transformed y values.
- It calculates the logarithm or exponential of the trend line.

3. ## Re: XY Graph and curve fitting (2003)

Thank you very much.

Federico.

4. ## Re: XY Graph and curve fitting (2003)

<hr>For a higher degree polynomial it's similar, but with more coefficients, and in general, the solution is found by iterative approximation<hr>

Approximations should not be needed unless the equation is non-linear in the coefficients...

Steve

5. ## Re: XY Graph and curve fitting (2003)

<img src=/S/blush.gif border=0 alt=blush width=15 height=15> You're correct - I wasn't thinking clearly.

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