We use Flot currently, but we're looking at purchasing a license for Emprise. Specifically, they offer candlestick charts, which are difficult to find elsewhere, yet absolutely critical in expressing the volatility of series data.
http://www.ejschart.com/
IMO, candlestick charts are way underutilized. Take application response time graphs for example. Most reporting suites rely on moving averages. If your application can have an acceptable average response time, but still have a high level of deviation. Let's say you have an app where, on average, one out of six requests is an entire order of magnitude slower than other requests:
# Ruby code where r is response in ms
r = [
100,
100,
100,
100,
100,
1000
]
r.inject{ |sum, ms| sum + ms }.to_f / r.size # => 250
So my average response time is 250ms, which I may consider acceptable, yet some responses are taking 1000ms, which I wouldn't consider acceptable. With a simple line chart, I wouldn't see the significant deviation. With a chandlestick chart, the upper boundary would be blindinly obvious.