border

     set pm3d border {retrace} {line-properties}
     set pm3d noborder

This option draws bounding lines around each pm3d quadrangle as it is rendered. Additional line properties (linetype, color, linewidth) are optional. By default the border is drawn as a solid black line with width 1.

set pm3d border retrace causes a border to be drawn in the same color as the quadrangle. In principle this should give the same result as noborder, but some output modes can suffer from antialiasing artifacts between adjacent filled quadrangles. Retracing the border hides these artifacts, at the cost of a larger output file.