Well for use at a high-volume website you would need something else. There's a limit of 50000 charts per day before it stops working and/or you are banned.
You might not want to generate the chart for each page view - you'd probably store/cache the png at your end.
Of course if you need to generate > 50K dynamic charts/day then you're right.
We (RescueTime) do use FusionCharts.... Nice system and a good value. Though each of the SWF files is 40-70k in size, which doesn't thrill me. I've yet to find a charting solution that does. :-(