There are three gnuplot commands which actually create a plot: plot,
splot and replot. plot generates 2-d plots, splot generates 3-d
plots (actually 2-d projections, of course), and replot appends its
arguments to the previous plot or splot and executes the modified
command.
Much of the general information about plotting can be found in the discussion
of plot; information specific to 3-d can be found in the splot section.
plot operates in either rectangular or polar coordinates -- see set polar
for details of the latter. splot operates only in rectangular coordinates,
but the set mapping command allows for a few other coordinate systems to be
treated. In addition, the using option allows both plot and splot to
treat almost any coordinate system you'd care to define.
splot can plot surfaces and contours in addition to points and/or lines.
In addition to splot, see set isosamples for information about defining
the grid for a 3-d function; splot datafile for information about the
requisite file structure for 3-d data values; and set contour and `set
cntrparam` for information about contours.