Undocumented behaviour of "smooth" options

[Ethan A M. <sf...@us...>]

On the bug tracker    <https://sourceforge.net/p/gnuplot/bugs/1911/>
Rainald Koch points out that when a data set contains some points with
invalid y values, the result of smoothing is unexpected. This is true
also if you use a filter to select a subset of the points, e.g.

    plot FOO using 1:( filter($2) ? $2 : 1/0) smooth acsplines

I can't find any mention of it in the documentation, but it seems that
this is intentional.  The "smooth" code has a pre-processing step that
splits the input data into N subsets separated by one or more "undefined"
points. The smoothing is then applied separately to each of the N segments
with no attempt to make the fit smooth or continuous at the junctions
between segments.

Is this documented somewhere that I haven't found?

Is there a recommended way to fit a single smooth curve to the
entire set of points after filtering?

Current CVS allows this work-around:

   set datafile missing NaN

but that is very recent.  Was there no way to handle this in the past?

	Ethan