JSXGraph
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Heat map
QR code 
<iframe 
    src="https://www.jsxgraph.org/share/iframe/heat-map" 
    style="border: 1px solid black; overflow: hidden; width: 550px; aspect-ratio: 55 / 65;" 
    name="JSXGraph example: Heat map" 
    allowfullscreen
></iframe>
This code has to 
<div id="board-0-wrapper" class="jxgbox-wrapper " style="width: 100%; ">
   <div id="board-0" class="jxgbox" style="aspect-ratio: 1 / 1; width: 100%;" data-ar="1 / 1"></div>
</div>

<script type = "text/javascript"> 
    /*
    This example is licensed under a 
    Creative Commons Attribution ShareAlike 4.0 International License.
    https://creativecommons.org/licenses/by-sa/4.0/
    
    Please note you have to mention 
    The Center of Mobile Learning with Digital Technology
    in the credits.
    */
    
    const BOARDID = 'board-0';

    const board = JXG.JSXGraph.initBoard(BOARDID, {
        boundingbox: [-5, 5, 5, -5],
        axis: true
    });
    var i, j, step, f,
        list = [];
    
    var c = board.create('slider', [[1, 4.5], [4, 4.5], [1, 1, 5]]);
    f = (x, y) => Math.sin(c.Value() * x) * Math.cos(c.Value() * y);
    
    step = 0.5;
    for (i = -4; i <= 4; i += step) {
        for (j = -4; j <= 4; j += step) {
            list.push(board.create('point', [i, j], {
                face: '[]',
                withLabel: false,
                highlight: false,
                strokeWidth: 0,
                name: '',
                size: (self) => 2 + 8 * Math.abs(f(self.X(), self.Y())),
                fillColor: (self) => (f(self.X(), self.Y()) >= 0) ? '#0000ff50' : '#ff000050'
            }));
        }
    }
 </script> 
/*
This example is licensed under a 
Creative Commons Attribution ShareAlike 4.0 International License.
https://creativecommons.org/licenses/by-sa/4.0/

Please note you have to mention 
The Center of Mobile Learning with Digital Technology
in the credits.
*/

const BOARDID = 'your_div_id'; // Insert your id here!

const board = JXG.JSXGraph.initBoard(BOARDID, {
    boundingbox: [-5, 5, 5, -5],
    axis: true
});
var i, j, step, f,
    list = [];

var c = board.create('slider', [[1, 4.5], [4, 4.5], [1, 1, 5]]);
f = (x, y) => Math.sin(c.Value() * x) * Math.cos(c.Value() * y);

step = 0.5;
for (i = -4; i <= 4; i += step) {
    for (j = -4; j <= 4; j += step) {
        list.push(board.create('point', [i, j], {
            face: '[]',
            withLabel: false,
            highlight: false,
            strokeWidth: 0,
            name: '',
            size: (self) => 2 + 8 * Math.abs(f(self.X(), self.Y())),
            fillColor: (self) => (f(self.X(), self.Y()) >= 0) ? '#0000ff50' : '#ff000050'
        }));
    }
}
<jsxgraph width="100%" aspect-ratio="1 / 1" title="Heat map" description="This construction was copied from JSXGraph examples database: BTW HERE SHOULD BE A GENERATED LINKuseGlobalJS="false">
   /*
   This example is licensed under a 
   Creative Commons Attribution ShareAlike 4.0 International License.
   https://creativecommons.org/licenses/by-sa/4.0/
   
   Please note you have to mention 
   The Center of Mobile Learning with Digital Technology
   in the credits.
   */
   
   const board = JXG.JSXGraph.initBoard(BOARDID, {
       boundingbox: [-5, 5, 5, -5],
       axis: true
   });
   var i, j, step, f,
       list = [];
   
   var c = board.create('slider', [[1, 4.5], [4, 4.5], [1, 1, 5]]);
   f = (x, y) => Math.sin(c.Value() * x) * Math.cos(c.Value() * y);
   
   step = 0.5;
   for (i = -4; i <= 4; i += step) {
       for (j = -4; j <= 4; j += step) {
           list.push(board.create('point', [i, j], {
               face: '[]',
               withLabel: false,
               highlight: false,
               strokeWidth: 0,
               name: '',
               size: (self) => 2 + 8 * Math.abs(f(self.X(), self.Y())),
               fillColor: (self) => (f(self.X(), self.Y()) >= 0) ? '#0000ff50' : '#ff000050'
           }));
       }
   }
</jsxgraph>
This heat map example relies heavily on the host element as parameter of attribute functions which was introduced in version 1.11.0: ``` board.create('point', [i, j], { size: (self) => 2 + 8 * Math.abs(f(self.X(), self.Y())), fillColor: (self) => (f(self.X(), self.Y()) >= 0) ? '#0000ff50' : '#ff000050' })); ``` With the parameter `self`, there is access to the properties of that point.
// Define the id of your board in BOARDID

const board = JXG.JSXGraph.initBoard(BOARDID, {
    boundingbox: [-5, 5, 5, -5],
    axis: true
});
var i, j, step, f,
    list = [];

var c = board.create('slider', [[1, 4.5], [4, 4.5], [1, 1, 5]]);
f = (x, y) => Math.sin(c.Value() * x) * Math.cos(c.Value() * y);

step = 0.5;
for (i = -4; i <= 4; i += step) {
    for (j = -4; j <= 4; j += step) {
        list.push(board.create('point', [i, j], {
            face: '[]',
            withLabel: false,
            highlight: false,
            strokeWidth: 0,
            name: '',
            size: (self) => 2 + 8 * Math.abs(f(self.X(), self.Y())),
            fillColor: (self) => (f(self.X(), self.Y()) >= 0) ? '#0000ff50' : '#ff000050'
        }));
    }
}

license

This example is licensed under a Creative Commons Attribution ShareAlike 4.0 International License.
Please note you have to mention The Center of Mobile Learning with Digital Technology in the credits.