spline [<range specification>] <function name> '<filename>' [index <index specification>] [every <every specification>] [using <using specification>]
The spline command fits a spline to a datafile. A special function is created that represents the spline fit and can be used in the same way as any other user-defined function. For example:
spline f() 'data.1'
would create a function that is a fit to the data in the file data.1. By default, the spline command uses the first two columns of a data file in a manner analogous to the plot command. The index, every and using modifiers can be used in the same way as in the plot command to select which parts of the datafile should be used; see the datafile section for more details.
Note that trying to generate splines of multi-valued functions will not, in general, produce useful results.