what is the use of "u" in tabulated output?

[<pl...@pi...>]

  HI,


I often use conditional using clauses to plot part of a range of data

plot datafile using   (($1<=1995.55)?$1:NaN):(2*fcos1($1))

I tried this when writing to a file specified with "set table"  and 
instead of cutting off the output, it appends garbage data with a "u".

help table tells me this means "undefined"

I see two problems here. I did not ask for undefined output , I 
specified NaN.

Secondly , what is the possible use of "undefined" values in a data file?

1995.46  1.00914  i
1995.54  1.10947  i
1.98547e-81  4.74363e+170  u
3.75845e+174  7.52884e-24  u
0  2.122e-314  u
0  0  u
0  0  u
1.9771e+161  1.44402e+214  u
-1.22337e-44  1.1735e-319  u
1.01887e+189  7.49746e+247  u
5.35166e+199  2.10124e+88  u


 From my using clause I would expect either

NaN [ correct value of 2*fcos($1) ] o
NaN [ correct value of 2*fcos($1) ] i

or preferable nothing at all , as would reflect the usual terminal 
output for NaN points.

  Version 4.7 patchlevel 0    last modified 2012-10-16
         Build System: Linux i686



Thanks , Peter