#2895 Data filter `if (...)` fails with `using 1:2`, works with `using ($1):2`

[Anonymous]

Bug description

An 'if' condition inside a plot command fails if the 'using' modifier is contains only plain column numbers.

Minimal example (corresponding to attached filebug.plt):

$D << EOD
1 1 10
2 2 20
3 1 30
EOD

plot $D u 1:3    w lp if ($2 == 1)   # fails (output below)
plot $D u ($1):3 w lp if ($2 == 1)   # plots (1,10) and (3,30)
plot $D u 1:($3) w lp if ($2 == 1)   # plots (1,10) and (3,30)

Output of the first plot command:

"bug.plt" line 7: warning: Skipping data file with no valid points

plot $D u 1:3    w lp if ($2 == 1)
                                     ^
"bug.plt" line 7: x range is invalid

Expected behavior

All three commands plot (1,10) and (3,30). The documentation example for the filter,
plot FOO using 2:3 with linespoints if (strcol(1) eq "ABC"), also uses plain
column numbers.

Platform and version

Gnuplot version: gnuplot 6.0 patchlevel 5

Distribution package on Manjaro Linux, installed through pacman.