[R] Interpolate through NAs using monotonic smoothing spline

[Jeff Newmiller]

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>From your description, you might consider monotonic spline fitting the diff() of your data and integrating the fitted result... but without your sample plots I could be way off.
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On October 23, 2014 9:35:28 AM PDT, Ingrid Charvet <Ingrid.Charvet at rms.com> wrote:
>In the figure attached "test", I would like to interpolate the empty -
>"NA" section to connect the two curves.
>
>1/ I tried using "na.approx" and "na.spline", without success.
>
>na.approx(test,na.rm=FALSE)
>na.spline(test,na.rm=FALSE)
>
>In the first case it just draws a straight line between the end and
>beginning points of each curve, respectively, which doesn't look
>smooth. In the second case the cubic spline interpolation results
>overshoot above 1.0 which I would like to avoid (see attached test.na)
>.
>
>2/ I have also used  splinefun for a monotonic increasing spline, which
>is the best result so far
>Yfun <- splinefun(X,test,method="monoH.FC")
>test.spline <- Yfun(X)
>
>however the resulting curve displays a "kink" at the limit of the
>interpolation zone (see attached figure test.splinefun).
>
>is there a way to constrain the spline in na.spline in 1/, for example,
>to force it to be monotonic strictly increasing?
>Alternatively can we alter the smoothing performed by splinefun somehow
>to avoid this "kink"?
>
>Any other suggestions welcome!
>Thank you
>IC
>
>
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