kalani's Tech blog: November 2009

Plotting a moving graph is nothing other than plotting an instance of a varying data set at each refresh. However, to get the moving effect you need to change the data set as stated below

1. Discard the leftmost Y value of the previous step
2. Shift the remaining Y values to the left
3. Add the new coming value as the rightmost Y value

Flot is an opensource Javascript plotting library for jQuery. Now let's see how to do this using Flot.

Firs you need to download flot library and add followings to your js folder.
1.jquery.js
2.jquery.flot.js
3.excanvas.js (This is needed if your browser is IE)

Now let's create the html page

<html>
<head>
    <!--Include the css file and flot libs-->
    <LINK REL=StyleSheet HREF="css/layout.css" TYPE="text/css">

    <script language="javascript" type="text/javascript" src="js/jquery.js"></script>
    <script language="javascript" type="text/javascript" src="js/jquery.flot.js"></script>
    <script language="javascript" type="text/javascript" src="js/excanvas.js"></script>
    ....
    ....                                                                 
    ....

Here is the CSS file used

body {
  font-family: sans-serif;
  font-size: 16px;
  margin: 50px;
  max-width: 800px;
  background-color:#000000;
}

div.graph {
    width:500px;
    height:300px;
}

Let's include javascript functions inside HEAD

....
....
<script id="source" language="javascript" type="text/javascript">
        var running = false;
        var array;
        var xscale = 100; 

        //this function does the plotting 
        function draw() {
           //shift the values to left
            for (var i = 0; i < this.xscale - 1; i++) {
                this.array[i] = [i,this.array[i + 1][1]];  // (x,y)
            }

            //add the new coming value to the last postion
            this.array[this.xscale - 1] = [this.xscale - 1,Math.random()];

                $.plot($("#graphdiv"), [
                    {
                        label: "Y vs X",
                        data: this.array,
                        lines: { show: true, fill: true, fillColor: "rgba(249, 28, 61,0.3)",lineWidth: 3.5 },
                        color:"rgba(249, 28, 61,1)"
                    }
                ],
                {
                    xaxis: {
                        ticks: generateTicks(),
                        min: 0

                    },
                    yaxis: {
                        ticks: [0 , 0.2, 0.4, 0.6, 0.8, 1.0, 1.2],
                        min: 0
                    },
                    
                    //placing a grid
                    grid: {
                        show: true,
                        color: '#474747',
                        tickColor: '#474747',
                        borderWidth: 1,
                        autoHighlight: true,
                        mouseActiveRadius: 2
                      }


                });
        }


        //This creates the data array with 0.0 inital Y values at the initialization time
        function initialize() {
            this.array = new Array();
            for (var i = 0; i < xscale; i++) {
                this.array[i] = [i, 0.0];
            }
        }

        //This is used to generate ticks for X axis (value 0 will be on the right)
        function generateTicks() {
            var tickArray = [];
            var startTick = 20;
            var i = startTick - 1;
            var weight = this.xscale / 20;
            do {
                var t = (startTick - i) * weight - 1;
                var v = i * weight;
                if (v == 0) {
                    v = "0";
                }
                tickArray.push([t, v]);
                i--;
            } while (i > -1);
            return tickArray;
        }

        //This function is called once per every 1000ms
        function refreshStat() {
            if (!running) {
                running = true;
                draw();
                running = false;
            }
        }

        $(document).ready(function () {
            initialize();
            refreshStat();
            setInterval("refreshStat()", 1000);
        });

    </script>
</head>

Now here goes the BODY of the html

<body>
<table cellspacing="50">
    <tr>
        <td>
            <div id="graphdiv" class="graph"></div>
        </td>
    </tr>
</table>
</body>
</html>

Then you'll get a moving graph. I've posted some still instances of the graph below

You can customize the graphs as your wish. Flot API will help you on that. This is pretty simple and straight. Just give a try.

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