splot 'datafile.dat'
where the file "datafile.dat" might contain:
# The valley of the Gnu. 0 0 10 0 1 10 0 2 10
1 0 10 1 1 5 1 2 10
2 0 10 2 1 1 2 2 10
3 0 10 3 1 0 3 2 10
Note that "datafile.dat" defines a 4 by 3 grid ( 4 rows of 3 points each ). Rows are separated by blank records.
Note also that the x value is held constant within each dataline. If you instead keep y constant, and plot with hidden-line removal enabled, you will find that the surface is drawn 'inside-out'.
Actually for grid data it is not necessary to keep the x values constant within an dataline, nor is it necessary to keep the y values the same along the perpendicular datalines. gnuplot requires only that the number of points be the same for each dataline.