The JSXGraph extension provides a function Numbas.extensions.jsxgraph.makeBoard which creates a JSXGraph board and wraps it inside an HTML div element to embed in the page.

The simplest use is to define a custom function in Javascript which returns an HTML value, like so:

// First, make the JSXGraph board.
// The function provided by the JSXGraph extension wraps the board up in
// a div tag so that it's easier to embed in the page.
var div = Numbas.extensions.jsxgraph.makeBoard('400px','400px',
    {boundingBox: [-13,16,13,-16],
     axis: false,
     showNavigation: false,
     grid: true

// div.board is the object created by JSXGraph, which you use to
// manipulate elements
var board = div.board;

// Then do whatever you want with the board....

// and return the container div
return div;

The question Using student input in a JSXGraph diagram is a more complete example of creating a JSXGraph diagram based on question variables and student input.

For help on using JSXGraph, see the official documentation.