Syntax:
set style fill {empty | solid {<density>} | pattern {<n>}}
{border {<linetype>} | noborder}
The default fillstyle is empty.
The solid option causes filling with a solid color, if the terminal
supports that. The 
density
parameter specifies the intensity of the
fill color. At a density of 0.0, the box is empty, at density of 1.0,
the inner area is of the same color as the current linetype.
Some terminal types can vary the density continuously; others implement
only a few levels of partial fill. If no density parameter is given,
it defaults to 1.
The pattern option causes filling to be done with a fill pattern supplied
by the terminal driver. The kind and number of available fill patterns
depend on the terminal driver. If multiple datasets using filled boxes are
plotted, the pattern cycles through all available pattern types, starting
from pattern n, much as the line type cycles for multiple line plots.
The empty option causes filled boxes not to be filled. This is the default.
It is equivalent to the solid option with a density parameter of zero.
By default, border, the box is bounded by a solid line of the current
linetype. border lt specifies that a border is to be drawn using
linetype lt. noborder specifies that no bounding lines are drawn.