As described by MsShorty: Sometimes when you have a
table of data, it is easier/preferable to approximate the function as a
series of straight lines connecting the data points (similar to what a
line chart/XY chart does) rather than attempt to fit the data to a
curve.
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These problem are solved by PUP6 - Power Utilities 6 at
http://www.j-walk.com/ss by $35 as a multiple add-ins to Excel, very usefull
that I bought.
I advise the purchase animatedly!