Interesting question. So if I use this to display site metrics for me and my team, is that a commercial use? I'm not selling that service but in that example I would be using it to provide a visual indication of how the site is doing which helps me do my job.

Generally pricing isn't particularly challenging, people exchange money for perceived value so identify the value that you add and charge for that.

So places where you might be adding value;

developer training - A common source of value and one which a lot of non-community FOSS members appreciate. So folks who aren't part of the community or know much about anything find it valuable to have someone who will answer their stupid questions in a timely fashion without insulting them. That could certainly be charged on a per-developer level and annually.

data-hoisting - (see what I did there? clever huh?) basically there is value in getting your data from X hosted on Y. So lets say I periodically scrape all of the stock prices for the DJIA and want to provide an interactive chart for that. A data hoisting service would provide a repository for post-processed and chart friendly data. You can price that with visualization support almost on a contract basis.

custom visualizations - there will be people of course who want to visualize something in a way which is unique, consulting services for them can be quite valuable.

ad-sponsored charts - something between hoisting and foisting but there are charts that people "check" very often (like the 5 day weather forecast, or the traffic delays on various freeways). You can certainly create pages which display visualizations of these and also are "sponsored" by an advertiser. The trades of bit coins for example, rent prices by zip codes, basically changing data sets of interest to both a number of people and to people who would like to advertise to people interested in that data.

In a world of so many wonderful free open source graphing libraries, it's hard to see any reasonable licensing option for this. It might help if there was a page that listed features that a developer could not accomplish for $300 in flot, highcharts, or ndv3, or other popular options.

Highcharts isn't free either, at least for commercial use.

It's cheaper.

Highcharts is cheaper if you use it for a single webpage (90$) but if you want to use if for a real web app it's more expensive (590$ for a single developer). Having said that, in my enterprise my dev colleagues use it and they think it's a great product.

EDIT: if you host it for a personal/non-profit project it is free according to Creative Commons Attribution-NonCommercial License

another product we've used in the past is Amcharts, good buy maybe not good as Highcharts

And I still prefer the free flot or flotr2, because they are easier to modify.

On the other hand, do I get a price break if I contribute to the GitHub project? What if I am a company?