Nigel,
A word of warning: using a general field can result in SLOOOOOWWWW response times when the graph size is large.

When you reach a point where it is taking too long, investigate Flipper. I can quickly (repeat quickly) generate graphs on the fly from any table.

VERY FAST. And, I can quickly switch from a bar chart to a pie chart, to a 3-d chart, etc, etc.