Charts/XY(scatter)/Smooth lines option

[IAN]

I would appreciate your help in the following issue:

We like to know what function the Excel developpers have used in Excel for
Charts/XY(scatter)/smooth lines. It iseems to us that it is a B-spline but
there are many of those around. Could you please provide us with the exact
spline function used for interpolation in this Excel option and possibly the
code

Thanks a lot in advance
Ian

 

[Jon Peltier]

Excel actually uses Bezier curves, though they can be approximated by
splines. Look for the Smooth_curve_bezier_example_file.xls download in the
bottom part of this page on Brian Murphy's web site:

http://www.xlrotor.com/resources/files.shtml

- Jon

 

[IAN]

Dear Jon,

Thanks for the message. I followed your advise but it seems that the code in
Brian's web site is only for 4 points. In Excel we can give any number of
points without restrictions and Excel gives us the smoth line.
I would appreciate your advise

Thanks in advance

Ian

 

[Jon Peltier]

What are you using the smoothed lines for? Not to interpolate values between
plotted points, I hope, because the curves hold no information about the
values between the points.

I haven't looked at Brian's workbook in a couple of years, so I forget how
he did it. Couldn't you extend the formulas, or loop more in the code, or
extend whatever technique he uses?

- Jon

 

[IAN]

Dear Jon,

I have a table of X and Y which are more than 4. Normally Bezier is for 4
points. By interpolating with Excel I can get a smooth line from several
input points but not the values in between. In Brians code you can in fact
take the interpolated values between the original interpolated points
according to a step. However it has as an input only 4 points. It seems
difficult to me how to extend it since it is not straight forward. I think
Brian should have done it and if he can help I would very much appreciate

Thanks again

ian

 

[Jon Peltier]

As I recall, two adjacent points in the chart form the outer pair of points
in the Bezier curve, and Brian includes algorithms for determining the inner
two. You need to do the analysis between every adjacent pair of points in
the series.

- Jon