3.9.1 Horizontally arranged data files
plot 'data.dat' index 1 using rows 1:2
plot 'data.dat' index 1 using columns 1:2
When plotting horizontally-arranged data files, the meanings of the index and every modifiers are altered slightly. The former continues to refer to vertically-displaced blocks of data separated by two blank lines. Blocks, as referenced in the every modifier, likewise continue to refer to vertically-displaced blocks of data points, separated by single blank lines. The row numbers passed to the using modifier are counted from the top of the current block.
plot 'data.dat' using rows 1:2 every 2::3::9
would plot the data in row 2 against that in row 1, using only the values in every other column, between columns 3 and 9.