set style fill border

The bare keyword border causes the filled object to be surrounded by a solid line of the current linetype and color. You can change the color of this line by adding either a linetype or a linecolor. noborder specifies that no bounding line is drawn. Examples:

     # Half-intensity fill, full intensity border in same color
     set style fill solid 0.5 border
     # Half-transparent fill, solid black border (linetype -1)
     set style fill transparent solid 0.5 border -1
     # Pattern fill in current color, border using color of linetype 5
     plot ... with boxes fillstyle pattern 2 border lt 5
     # Fill area in cyan, border in blue
     plot ... with boxes fillcolor "cyan" fs solid border linecolor "blue"

Note: The border property of a fill style only affects plots drawn with filledcurves in the default mode (closed curve).