Real-time graphing

This little script shows how to graph the data from a sensor in a web browser. Using AJAX, the web browser retrieves once per second data from the web server by calling the script fakesensor.php.

Beware that in order to simplify the handling of AJAX we use the additional library prototype.js in this example.

Here, fakesensor.php just returns a random number, but one can image that real sensor data can be accessed similarly.

The underlying JavaScript code

  <script type="text/javascript" src="http://jsxgraph.uni-bayreuth.de/distrib/prototype.js"></script>
  <style>
    #jxgbox {
       background-color:black;
    }
  </style>

var brd, g, 
    xdata = [], ydata = [],
    turt,i;

brd = JXG.JSXGraph.initBoard('jxgbox', {axis:true, boundingbox:[0,20,30,-2]});

// Draw the additional lines
turt = brd.create('turtle',[],{strokecolor:'#999999'});
turt.hideTurtle().right(90);
for (i=5;i<=15;i+=5) {
    turt.penUp().moveTo([0,i]).penDown().forward(30);
}


fetchData = function() {
    new Ajax.Request('/ajax/fakesensor.php', {
        onComplete: function(transport) {
            var t, a;
            if (200 == transport.status) {
                t = transport.responseText;
                a = parseFloat(t);
                if (xdata.length<30) {          
                    xdata.push(xdata.length); // add data to the x-coordinates, if the right border has not been reached, yet
                } else {
                    ydata.splice(0,1);        // remove the oldest entry of the y-coordinates, to make the graph move.
                }
                ydata.push(a);
                if (!g) {                   // If the curve does not exist yet, create it.
                    g = brd.create('curve', [xdata,ydata],{strokeWidth:3, strokeColor:'yellow'}); 
                } 
                brd.update();
            };
    }});
};
setInterval(fetchData,1000);  // Start the periodical update

The PHP code of fakesensor.php

<?php
   echo mt_rand(0,20);
?>