cronで15分毎にload averageを取得して、それをgnuplotでpngのグラフに書き出すrubyスクリプト gnuplotが必要です。 sudo apt-get install gnuplot cronへの登録例（15分毎の実行） */15 * * * * /var/www/loadaverage.rb

loadaverage.rb

#!/usr/bin/env ruby

# please install gnuplot
# sudo apt-get install gnuplot

# crontab
# */15 * * * * /var/www/loadaverage.rb

logname = "/var/www/loadaverage/#{Time.now.strftime('%Y%m%d')}.log"

pngname = "/var/www/load.png"

File.open("#{logname}", "a") do |f|
  f.puts((Time.now+32400).strftime("%s") + " " + `uptime`.split("average: ").last.split(", ")[2])
end

script = <<'EOS'.sub("PNGNAME", "#{pngname}").sub("LOGNAME", "#{logname}")
set terminal png
set output "PNGNAME"
set xdata time
set timefmt "%s"
set grid y
set format x "%H"
set title "Load Average (Daily)"
set size 0.8, 0.5
plot "LOGNAME" using 1:2 title "Load" with lines
EOS

system("echo '#{script}' | gnuplot")