set terminal png
set title font "Arial,20" 'FNAME'
set xlabel font "Arial,20" 'Time'
set ylabel font "Arial,20" 'EW(nm)' textcolor rgb 'blue' offset 1,0
set y2label font "Arial,20" 'EW(nm)' textcolor rgb 'green'
set xtics rotate by -90
set y2tics
set grid
set xzeroaxis lt 1
unset key
set xdata time
set timefmt "%Y-%m-%d %H:%M:%S"
set format x "%H:%M"
set output "tmp.png"
plot "FILENAME" u 1:15 w l lt rgb 'blue' axis x1y1
set output "tmp.png"
replot "FILENAME" u 1:16 w l lt rgb 'green' axis x1y2
!mv tmp.png ewlog.png