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How do I add one xy plot to a nother (to one plot NOT two)

  1. #1
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    How do I add one xy plot to a nother (to one plot NOT two)

    Hie everyone

    I have two sets of x and y data according to below.
    0 68689.04369
    0.2 128722.866
    0.400 195249.1997
    0.600 264022.7596
    0.800 332491.5572
    1.000 399157.9391
    1.200 463178.1046
    1.400 524112.3796
    1.600 581770.108
    1.800 636113.5069
    2.000 687198.0156
    &
    0.000 380030.817
    0.164 547092.430
    0.328 689505.197
    0.491 813700.023
    0.655 923545.080
    0.819 1021349.438
    0.983 1108502.716
    1.147 1185881.142
    1.311 1254104.252
    1.474 1313694.364
    1.638 1365170.748
    1.802 1409098.614
    1.966 1446106.456
    2.130 1476881.628

    How do I get the sum of these two curves when the data points dont match up on the x values.???

    I have tried some different options but havenīt struck gold yet

    Thanks in advance

  2. #2
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    Re: How do I add one xy plot to a nother (to one plot NOT two)

    What options have you tried?

    In cases like this, I see two basic possibilities: 1) Use an interpolation algorithm to find y(s) at specific x's for each curve, which you can then add up. 2) Use a regression algorithm to fit each curve to an equation, then add those two equations together. I can't say which approach will be more suitable to the specific data you have. I would probably lean towards the regression/curve fitting approach, which should be pretty easy to program into Excel using the LINEST() function.

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    Re: How do I add one xy plot to a nother (to one plot NOT two)

    Hie

    Thats pretty much what I have been trying. But I dont like that itīs a linear interpolation between the data point. I was thinking of a function that can estimate a x degree polynom and then I can add these two polynoms to each other and voila.

    How do I get a x degree polynom fitted to certain datapoint whitout haveto go thru the chart and use trendlines. ( I need the this to be automatic as I will vary the input a lot)

  4. #4
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    Re: How do I add one xy plot to a nother (to one plot NOT two)

    Use the LINEST() function. It is very poorly explained by the built in help file, but the LINear in LINEST means linear in the sense that you learn in linear algebra. Since all polynomials are linear functions, LINEST can be used to regress a polynomial.

    The usual syntax (for a 3rd order polynomial for example) would be =LINEST(known_y's,known_x's^{3,2,1}). Remember that LINEST is an array function and must be confirmed using ctrl-shft-enter.

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