What language does have built-in plotting capabilities that don't depend on external libraries?

It depends on how you define "built-in" exactly. I would argue that in this context, if the end user isn't aware that an external library is used under the hood, it still qualifies as built-in. In which case, R has plot() in its stdlib.

Language vs system with a language?

Mathematica, matlab, maple, octave, etc.