[R] Interpolate through NAs using monotonic smoothing spline

[Ingrid Charvet]

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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