Code:
while true; do wget -qO - "http://www.bitcoincharts.com/charts/chart.png?width=940&m=mtgoxUSD&SubmitButton=Draw&r=5&i=&c=0&s=&e=&Prev=&Next=&t=S&b=D&a1=&m1=10&a2=&m2=25&x=0&i1=SStoch&i2=MACD&i3=AccDist&i4=&v=1&cv=0&ps=0&l=0&p=0&" | convert - -negate - | feh --bg-center -; sleep 900; done
(of course you may customize this modifying the url)it needs the last version of feh (for images from stdin), but you may use other visualizer if you want (eg xv -rv -rbg black -rmode 5 -root -quit -).
the only problem is the 15m time phase; polling would be rude...
screenshots:
