oddity in putting a conditional in the using clause

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

Hi,

I seem to have an oddity in putting a conditional in the using clause in 
v5.0


  tail longest.dat

171  3342.447
172  3371.391
173  3400.336
174  3465.342
175  3458.227
176  3324.896
177  3389.902
178  3400.818
179  3429.764
180  3476.738


G N U P L O T
         Version 5.0 patchlevel alpha    last modified 2014-05-10




gnuplot>  plot  "longest.dat" using (($1/2*2==$1)?NaN:$1):2

          x range is invalid


I'm trying to plot for the even  values in col 1.
if I do :  using (($1/2*2==$1)?$1:NaN):2   it plots every point
and  using (($1/2*2==$1)?NaN:$1):2 complains that it has an empty x 
range ( ie using is always returning NaN.

The syntax of my using clause seem right if I test it in a print:

gnuplot>   i=179;print (i/2*2==i)?i:NaN
NaN
gnuplot>   i=178;print (i/2*2==i)?i:NaN
178

I know I could do this using "every" with that sample data but this is 
about the syntax not giving expected result.

Peter.