Excel - problem with trendlines

breadmonkey

I have a bunch of data points from some software and I need to interpolate between them to find the values at different points.

The way I'm trying to do this is as follows: I'm plotting the points I have on an XY scatter in Excel, adding a trendline and using the interpolation function for the trendline to calculate the values at the chosen points.

For some reason, the interpolation functions Excel gives me don't work. See below where I have used the equations on the chart to try to plot the line (to check if it worked). You can see that for the line above the x axis, the plot of the interpolation function quickly deviates and for the line below the x axis, it's slightly better but is still somewhat off. I am *certain* that I have input the correct function.



Does anyone know what might be causing this?

EDIT: I have intentionally only tried to plot the interpolation functions for half of the total length along the x axis.

Thanks

5uspect

You have to increase the number of digits in the returned equation a lot. Excel is retarded like that.

Have you any rational for not using linear interpolation?