The `set datafile missing` command allows you to tell `gnuplot` what character string is used in a data file to denote missing data. Exactly how this missing value will be treated depends on the using specifier of the `plot` or `splot` command.
Syntax:
set datafile missing {"<string>"}
show datafile missing
unset datafile
Example:
# Ignore entries containing IEEE NaN ("Not a Number") code
set datafile missing "NaN"
Example:
set style data linespoints
plot '-'
1 10
2 20
3 ?
4 40
5 50
e
set datafile missing "?"
plot '-'
1 10
2 20
3 ?
4 40
5 50
e
plot '-' using 1:2
1 10
2 20
3 ?
4 40
5 50
e
plot '-' using 1:($2)
1 10
2 20
3 ?
4 40
5 50
e
The first `plot` will recognize only the first datum in the "3 ?" line. It will use the single-datum-on-a-line convention that the line number is "x" and the datum is "y", so the point will be plotted (in this case erroneously) at (2,3).
The second and third `plot` commands will correctly ignore the middle line. The plotted line will connect the points at (2,20) and (4,40).
The fourth `plot` will also correctly ignore the middle line, but the plotted line will not connect the points at (2,20) and (4,40).
There is no default character for `missing`, but in many cases any non-parsible string of characters found where a numerical value is expected will be treated as missing data.