There's a need for a good charting library built on top of d3. Kind of like Highcharts, in terms of usability, but free. d3 is powerful but not as easy to use and customize as Highcharts.
Can I suggest http://www.sibdo.com For individuals it's free and built on top of d3 with some extra functionality that Higcharts does't have. You can even drag files directly onto to the visualizations and the data will render. Also really nice UI for mobile.
Looking at the Sibdo pricing page, it looks like much higher pricing (compared to the more established competitors) at $95 a month for use on a SINGLE website with a confusing limitation to "50 users" whatever that means.
Not only that, the example graphs and charts look very basic.