Quadratic from three intercepts

Template for JSXGraph

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/jsxgraph/0.99.7/jsxgraphcore.js"></script>
<link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/jsxgraph/0.99.7/jsxgraph.css">
<div id="jxgbox" class="jxgbox" style="width:500px; height:500px">
</div>

JavaScript

const board = JXG.JSXGraph.initBoard('jxgbox', {
boundingbox: [-5, 5, 5, -5],
axis: true
});
var el1 = board.create('line', [
[0, 0],
[1, 0]
], {
strokeOpacity: .2,
strokeColor: '#000000',
fixed: true,
name: 'y=0'
});
var el0 = board.create('line', [
[0, 0],
[0, 1]
], {
strokeOpacity: .2,
strokeColor: '#000000',
fixed: true,
name: 'x=0'
});
var xi1 = board.create('glider', [2, 0, el1], {
name: 'X-intercept2',
visible: true,
snapToGrid: true,
snapSizeX: 0.1,
snapSizeY: 0.1
});
var xi2 = board.create('glider', [-2, 0, el1], {
name: 'X-intercept1',
visible: true,
snapToGrid: true,
snapSizeX: 0.1,
snapSizeY: 0.1
});
var yi = board.create('glider', [0, 1, el0], {
name: 'Y-Intercept',
visible: true,
snapToGrid: true,
snapSizeX: 0.1,
snapSizeY: 0.1
});
// Macro function plotter
function addCurve(board, func, atts) {
var f = board.create('functiongraph', [func], atts);
return f;
}

// Simplified plotting of function
function plot(func, atts) {
if (atts==null) {
return addCurve(board, func, {strokewidth:2});
} else {
return addCurve(board, func, atts);
}
}

function f(x) {
var q = (-1)*parseFloat(xi1.X());
var p = (-1)*parseFloat(xi2.X());
var r = parseFloat(yi.Y());
var a = r / (p * q);
var b = (r * (q + p)) / (p * q);
var c = r;
var func = a * x * x + b * x + c;
return func;
}

c = plot(f);