This one was a real bear! I had to use View Source and go through a lot of stuff to find their <img src> tag that was displaying the graph I assume you wanted. With some sites/charts it's hard to justify the effort.
Also, your attempt is a good example of what I was talking about where you have to use the URL to the chart itself; not the URL to the page that contains the chart.
I used (without spaces): [ chart ]chart.bigcharts.com/bc3/quickchart/chart.asp?symb=%24compx&compidx=aaaaa%3A0&ma=0&maval=9&uf=0&lf=1&lf2=0&lf3=0&type=2&size=2&state=8&sid=3291&style=320&time=7&freq=1&nosettings=1&rand=975&mocktick=1&rand=8915[ /chart ]
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