我正在寻找一种在 gnuplot 中绘制水平线（或“阴影”）以下图形以显示白天与黑夜的方法：
生成此图像的脚本是：
set autoscale
set title "Campo magnetico terrestre - Intensidad - Ultimas 24hs"
set title font "Freesans, 11"
set timefmt '%Y-%m-%d %H:%M:%S'
set xdata time
set xtics font "Freesans, 9"
set ytics font "Freesans, 9"
set xlabel font "Freesans, 9"
set ylabel font "Freesans, 9"
set key font "Freesans,9"
set key left center
set xlabel "Fecha/Hora"
set ylabel "Campo (mG)"
set terminal png enhanced size 1200,600
set output "dia.png"
set grid y
set grid x
set datafile separator '|'
plot "< sqlite3 /data/magneto/magneto.db 'select * from v_dia'" using 1:2 title 'X' with lines, \
"< sqlite3 /data/magneto/magneto.db 'select * from v_dia'" using 1:3 title 'Y' with lines, \
"< sqlite3 /data/magneto/magneto.db 'select * from v_dia'" using 1:4 title 'Z' with lines, \
"< sqlite3 /data/magneto/magneto.db 'select * from v_dia'" using 1:5 title 'Total' with lines
你能给我一些帮助吗？提前致谢！