I'm not very ambitious I'm afraid....
I was afraid that was the case
I think the right way to do long term graphs or fancy post-processing of collected data is via the so called database logging feature using something like Excel or perhaps by enabling a graph itself to log its data to file. Graphs arn't free from a perf perspective. They take memory and cpu cyles to update. Early on, I implemented code so that only those graphs currently being displayed get updated so as to not take the perf hit for all graphs all the time. I don't think taking the perf hit for all sims for all graphs all the time just in case someone may want to look at historical info in the future is a good idea.
I was afraid that that also would be the case. And probably savegame size.
The things I think are reasonable to do (meaning I'll probably implement them) are graph "features" (e.g. buttons to elect to show/not show zero valued series, change series color, etc.) additional graph types as requested, Internet mode enablment or Internet mode versions of current graphs where appropraite,...
These all seem to be basic but very useful features, especially being able to change almost white, imperceptible (woah, quite a word - I had to use a dictionary for that one) colour to something more useful.
...mutliple graph instances of the same type that can run at different update intervals, more in-memory data points per graph for longer histories (say 1000 instead of the current 300) and decent long term logging to comma seperated format for off-line graphing and manipulation via excel or similar either via the current logging paradym or by logging enablment on a per graph basis. I like that latter since it gives us a paradym for choosing what data we want to calculate and save (and what we don't) and at what interval.
This would pretty much remove the need for continuous graphing. Being able to choose different update intervals for same data does exactly the same thing I was hoping for (even more if combined with ~1000 or more data points). It would allow graphs for long term and also for current events. Also, automated interval based logging for user defined data would be interesting. Everyone could construct the graphs they wanted to based on saved data. Hard drive space could be an issue here, but if it was optional it would be okay, especially when hard drives have become generally Huge.
The things I am unlikely to implement anytime soon include a graphing scripting language, super flexible runtime graphs with mix and match data types, that kind of thing.
Well those would anyway be something extra, like cacao powder on top of whipped cream on top of hot chocolate. Mmm... Not really necessary (though it'd taste damn good).
A couple of things you didn't mention but would be nice to have are sort of grid of values of graph on y axis and timeline for x axis and... Well, for now that'd be everything I can come up with graphs.