Syntax:
plot 'file' index <m>{{:<n>}:<p>}
Data sets are separated by pairs of blank records. index 4#4m5#5 selects only set 4#4m5#5; index 4#4m5#5:4#4n5#5 selects sets in the range 4#4m5#5 to 4#4n5#5; and index 4#4m5#5:4#4n5#5:4#4p5#5 selects indices 4#4m5#5, 4#4m5#5+4#4p5#5, 4#4m5#5+24#4p5#5, etc., but stopping at 4#4n5#5. Following C indexing, the index 0 is assigned to the first data set in the file. Specifying too large an index results in an error message. If index is not specified, all sets are plotted as a single data set.
Example:
plot 'file' index 4:5
For each point in the file, the index value of the data set it appears in is
available via the pseudo-column column(-2). This leads to an alternative way
of distinguishing individual data sets within a file as shown below. This is
more awkward that the index command if all you are doing is selecting one
data set for plotting, but is very useful if you want to assign different
properties to each data set. See lc variable (p. ).
Example:
plot 'file' using 1:(column(-2)==4 ? $2 : NaN) # very awkward plot 'file' using 1:2:(column(-2)) linecolor variable # very useful!
http://www.gnuplot.info/demo/multimsh.html