Highcharts provides 20 different types of [charts](http://www.highcharts.com/docs/chart-and-series-types/chart-types) to choose from, and they make it simple to [combine](http://www.highcharts.com/docs/chart-and-series-types/combining-chart-types) chart types.
Ranked in these QuestionsQuestion Ranking
Pro Excellent browser support
Highcharts supports all modern browsers (including IE6 and up), iOS and Android.
Pro Lots of good examples
The site has a selection of good examples that will help you get started quickly.
Pro Good documentation
Very handy with examples and explanations.
Pro Library to support Microsoft .NET
Compatable with .NET framework 4+, can develop graphs from server side for ASP .NETand ASP .NET MVC applications.
Pro Supports multiple Y axes
Important for trend correlation, eg. comparing number of clicks to sale amounts. Not same scales / units.
Pro Seems to be quite stable
Even the most complicated GUIs are bug-free most of the time.
Pro Extremely simple to implement.
To get started is extremely simple. It has a low learning curve.
Pro A lot of widgets
Webix is one of the most extensive UI component libraries, second only to Sencha ExtJS. Not only considering the number of widgets, but also the API methods for manipulating these widgets.
Pro Great support from the webix team
The company behind webix is really quick in answering any questions you have on their forum or via email.
Pro Webix Jet
The webix Jet library adds all the required features for SPA development (routing, template loading, ...)
Highcharts is not free for commercial projects, with a starting price of US$590 for Single Developer + Maintenance & Support
Con The examples are not that great
For instance, example's JsFiddle may get stalled when loading.
Con Not very popular
Not really a reason to not recommend it. But it has still a small user base. It deserves a lot more attention.
Con Not modular
The library is not modular (except for some additional more complex widgets). If you only need a few widgets, you still need to include the entire library.
Con Commercial license
It's not free for commercial applications.