[tlb]

Is it reliable to add methods to a built-in object like CanvasRenderingContext2D? I avoid it, but perhaps just out of superstition.

[makepanic]

You're right, extending the prototype of builtin objects is almost always a bad idea.

See String.prototype.contains → String.prototype.includes because mootools extended the String prototype[1]

[1] - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

[MJSplot_author]

If you know what you are doing it's fine, but it might cause incompatibility issues from namespace collisions.

You can rearrange the code to accept a context as the first argument if you are worried.