Licensing is per engineer, you require one license per engineer developing against the mxGraph API. The full text of the current license is
here. By purchasing mxGraph at the listed prices, you agree to the mxGraph license and support terms.
Firefox 24 and later, Microsoft Internet Explorer 9 and later, Safari 5.0 and later, Opera 15.0 and later, Android browser on Android 4.0 and later, iOS native browser on iOS 6 and later and Google Chrome 20.0 and later on all operating systems officially supported by these browsers and versions.
Support for Internet Explorer 8 and earlier is only available under custom licensing arrangements, Please contact us for more details.
No, under the vast majority of deployments there are no royalty fees. As long as your application that uses mxGraph isn't
a developer library or framework (i.e. is an end user application), the standard licensing applies with no additional fees.
If you wish to be sure please contact us.