Plotly.js is free now?

Account manager says: "I want to be able to view charts for all our sales data by time, with the ability to zoom in on a range... and make it pretty too!"

Developer [if smart] says: "Well, there's highcharts but it's kinda clunky and also with a premium pricing tier... the best way is always to roll your own SVG + animations with D3, but there are also a ton of HTML5/JS open source canvas libraries that could make some really cool charts... but without feature X, unless we took a week to build Y."

CEO Says: "This is a waste of time, <$1,000 in a tax-deductible expense doesn't matter if we can use this data in our business immediately without wasting more developer time on the issue."

Honestly there's a huge market for data-graphic libraries/services, and the most robust API's are all commercially licensed in some way. The multitude of free offerings all come VERY close to being great, but slightly miss the mark meaning it's "roll-your-own" for non-standard solutions.

Essentially: if the time their chart API's save your developers is > the cost of the license and you can set aside your purist ideals and undying love for d3.

[edit] I say that having assumed all 3 roles in said hypothetical discussion -- as of April 2015 I had yet to find an OS JS library with every chart type/feature we needed. Sometimes you can afford to do it right, other times the convenience is (begrudgingly) worth the cost.