Dataviz site Plotly has released a version of their JavaScript library for free use, as long as "all links and references to Plotly remain visible and unmodified."
Plotly.js JavaScript Graphing Library Basic Charts includes options such as bar charts, time series and bubble charts. More advanced plotting such as maps and 3D graphics require a paid version of the library. "We've decided to give it away for public use, then do scientific and 3D for a license," Plotly CEO Matt Sundquist said in an email. Free charts include options to create subcharts by a category, multiple chart types in one, and streaming charts with live data. However, some plots that I would consider basic, such as histograms and box plots, require the paid license.
Unlike the Google Chart API, Plotly's free library can be downloaded for use on your own website, so you don't have to worry about unanticipated changes or deprecation of the service. There is also a CDN option if you want to link to their version of the library instead of hosting the JavaScript library file on your own site.
A license for all 2D charts as well as the right to remove Plotly links and references costs $249 per developer per year. For access to the 3D graphing, it's $995 per developer per year. The annual fee as opposed to a one-time purchase is important to note if you're considering a paid version, since you'd likely be in license violation if you later decide not to renew -- unless you get rid of all your graphics created with the advanced versions.
In contrast, one of my favorite JavaScript libraries, Highcharts, requires a paid version for any commercial use -- but you only have to pay a one-time fee, not annually, unless you want support or new-version upgrades. Cost is only $90 for use on a single website; full-fledged multi-site and application licenses are $390 per developer, but that's a one-time charge and not annual. However, not all scientific charting options in Plotly would be available with Highcharts; and mapping is a separate new products, Highmaps, requiring another paid license for commercial use.
Below is a sample Plot.ly graph using mostly default options plus some additional code to make series names display correctly. The Plotly library does not appear to offer responsive design (changing graphic size as the browser window resizes) with basic graph code.
And, here is the same data graphed with the Highcharts library:
Both libraries offer the ability to click series on and off, which is useful in a case like this where one series has smaller totals than the others. Desktop and laptop viewers can also mouse over the graph to see data points. Tapping on an iPad works for both libraries, although I had trouble getting data points from Plotly's library when tapping on an Android phone. Plotly offers pan and zoom options for its graphs as well.
The Plotly library will likely appeal most to users of the Plotly site who would like the ability to generate similar visualizations on their own websites. If the Plotly look-and-feel appeals to you, you don't mind links back to their site (or you're willing to pay every year for the privilege of removing them) and the available visualization options meet your needs, this might be worth a look. If not, check out my chart of other available free tools for data visualization and analysis.