Yes, we use Gruff for our charts on SugarStats.com for the last few
months but we're about to update and move to a flash based graphing
system (amCharts or FusionCharts).
Gruff has been great for basic things. Another would be Scruffy which
is similar to gruff but seems a bit easier and more flexible to work
with. It also has graph combining options. Both use ImageMagick/
RMagick so they do server side proccess and take up resources.
ImageMagick/RMagick from what I see/hear is notorious for leaking
memory. I also think both solutions are a bit rigid in the amount of
visual customization you can do with the graphs without extending each
library and customizing to your own needs.
Sparklines looks cool for its specific purpose, I'mma have to check it