## Create an Interpolation Spline

## Usage

    interpSpline(obj1, obj2, bSpline, period, na.action)

## Arguments

`obj1` | Either a numeric vector of `x` values or a formula.
`obj2` | If `obj1` is numeric this should be a numeric vector of the same length. If `obj1` is a formula this can be an optional data frame in which to evaluate the names in the formula.
`bSpline` | If `TRUE` the b-spline representation is returned, otherwise the piecewise polynomial representation is returned. Defaults to `FALSE`.
`period` | An optional positive numeric value giving a period for a periodic interpolation spline.
`na.action` | a optional function which indicates what should happen when the data contain `NA`s. The default action (`na.omit`) is to omit any incomplete observations. The alternative action `na.fail` causes `interpSpline` to print an error message and terminate if there are any incomplete observations.

## Description

Create an interpolation spline, either from `x` and `y` vectors, or from a formula/data.frame combination.

## Value

An object that inherits from class `spline`. The object can be in the B-spline representation, in which case it will be of class `nbSpline` for natural B-spline, or in the piecewise polynomial representation, in which case it will be of class `npolySpline`.

## Author(s)

Douglas Bates and Bill Venables

## See Also

`splineKnots`, `splineOrder`, `periodicSpline`

## Examples

    data( women )
    ispl <- interpSpline( women$height, women$weight )
    ispl2 <- interpSpline( weight ~ height,  women )
    # ispl and ispl2 should be the same
    plot( predict( ispl, seq( 55, 75, len = 51 ) ) )
    plot( ispl )    # plots over the range of the knots
    splineKnots( ispl )

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