The first one is plot a line with background color.
reset set term png font ",20" xffffff #set png terminal and the background color is white set output "blank.png" set key off set ytics 0.5 plot sin(x) with line linecolor rgb"#ffffff" set output
The second one is plot something out of yrange.
set term png font ",20" set output "blank.png" set yrange [-1:1] set key off plot 2 with lines set output
|Plot nothing using gnuplot|