"The latest full version of plotly.js is available for free, unlimited use in any project" under the MIT open-source license, according to an announcement scheduled for release this morning.
In addition, "Plotly can now be used 100% offline in RStudio, MATLAB or [Python] Jupyter notebook free-of-charge," according to the statement. While Plotly clients for R, Python, and MATLAB have always been open-source, "the core graphics layer—plotly.js—was closed-source. Now the entire stack is open-source."
This marks a significant turnaround for the company, which had allowed free use of its library with some types of plots but charged $249/year per seat for more sophisticated statistical offerings and $999 per user per year to create 3-D charts. "Licensees who have previously purchased a plotly.js commercial license will no longer be charged the annual license fee and are encouraged to update to the latest plotly.js version under the MIT license," the company said. Plotly also still has a free public and paid private cloud hosting service.
You can see an example of an interactive Plotly graphic below: