jsxgraph

How do I rotate a piece of a function about a point, using JSXGraph?


I'm instructing students online and trying to describe the shape of a cubic function. As part of that, I want to emphasise the rotational symmetry of a cubic function around the point of inflection, using an interactive JSXGraph plot of y=x^3. I want to show that the section of the function where x<0, when rotated 180 degrees, maps directly onto the section of the function where x>0.

I have tried to accomplish this by creating 3 functiongraph elements:

f1, which is y=x^3 from -6<x<6, for the rotated function to overlap onto f2, which is y=x^3 from -6<x<0, the function I want to rotate rotatedf2, which is y=x^3 from -6<x<0, but transformed by rotation using a slider

However, the rotatedf2 function is never visible - nothing changes when using the slider.

I'm wondering if the issue is that I can't transform a functiongraph element, that I'd need to convert it to some other element such as curve.

Here is the code I've used:

    <div id="cubic11" class="jxgbox" style="width:400px; height:300px; margin:auto "></div>
    <script type="text/javascript">
    // Create board template
    JXG.Options.text.useMathJax = true;
    (function() {
        var board = JXG.JSXGraph.initBoard('cubic11', {
            boundingbox: [-3, 4, 3, -4],
            axis: true,
            keepaspectratio: true,
            showGrid: true
        });

        // Create function plot for f1: y = x^3
        var f1 = board.create('functiongraph', [function(x) {
            return x * x * x;
        }, -6, 6], {
            strokeColor: '#EF233C'
        });

        // Create function plot for f2: y = x^3 (between -6 and 0)
        var f2 = board.create('functiongraph', [function(x) {
            return x * x * x;
        }, -6, 0]);

        // Label point of inflection/rotation
        var ip = board.create('point', [0, 0], {
            fixed: true,
        });

        // Create slider for rotation of f2
        var r = board.create('slider', [
            [1, -2],
            [4, -2],
            [0, 0, 360]
        ], {
            name: 'rotation',
            snapWidth: 45,
            highline: {
                strokeColor: '#ED7D31'
            },
            label: {
                strokeColor: '#ED7D31'
            },
            fillColor: '#ED7D31',
            highlightStrokeColor: '#ED7D31'
        });

        // Create a new function graph rotated by the value of the slider for f2: y = x^3
        var t = board.create('transform', [function() {
            return r.Value() * Math.PI / 180
        }, ip], {
            type: 'rotate'
        });
        var rotatedf2 = board.create('functiongraph', [f2, t], {
            strokeColor: '#2ECC40'
        });

        // Inset equation text
        var text = board.create('text', [-4.5, -2.5, function() {
            var equationText = 'y=x^3';
            return '$$' + equationText + '$$';
        }], {
            fontSize: 14
        });
    })();
    </script><br>`

Any help is appreciated (AI was hopeless).


Solution

  • So, I was able to find an older question about line transformations that had an answer: JSXGraph: Applying a line transformation with applyOnce() Adapting Alfred's answer, here's my new code that works as intended:

    <div id="cubic1" class="jxgbox" style="width:400px; height:300px; 
    background-color:#FFFDE1; margin:auto "></div>
    <script type="text/javascript">
    // Create board template
    JXG.Options.text.useMathJax = true;
    (function() {
        var board = JXG.JSXGraph.initBoard('cubic1', {
            boundingbox: [-3, 4, 3, -4],
            axis: true,
            showCopyright: false,
            showNavigation: false
        });
    
        // Create function plot
        var f1 = board.create('functiongraph', [function(x) {
            return x * x * x;
        }, -3, 3], {
            strokeColor: '#EF233C'
        });
        var f2 = board.create('functiongraph', [function(x) {
            return x * x * x;
        }, -3, 0]);
    
        // Label point of inflection/rotation
        var ip = board.create('point', [0, 0], {
            fixed: true,
        });
    
        // Create slider for rotation of functiongraph
        var r = board.create('slider', [
            [0.5, -2],
            [2, -2],
            [0, 0, 180]
        ], {
            name: 'rotation',
            snapWidth: 45,
            highline: {
                strokeColor: '#ED7D31'
            },
            label: {
                strokeColor: '#ED7D31'
            },
            fillColor: '#ED7D31',
            highlightStrokeColor: '#ED7D31'
        });
    
        // Rotate functiongraph
        var rot = board.create('transform', [function() {
            return r.Value() * Math.PI / 180
        }, ip], {
            type: 'rotate'
        });
        var rotatedf2 = board.create('curve', [f2, rot], {
            strokeWidth: 2,
            strokeColor: '#ED7D31',
            fixed: false
        });
        rot.bindto(f2);
    
        // Inset equation text
        var text = board.create('text', [-4.5, -2.5, function() {
            var equationText = 'y=x^3';
    
            return '$$' + equationText + '$$';
        }], {
            fontSize: 14
        });
    })();
    </script><br>