Matthew Knott

This site is a self-contained low down on what's going on in my life, what I'm working on, what I'm thinking about, and how I'm feeling about life in general.

Friday, 25 April 2008

Canvas Based Bar Graph. Part 2

I recently wrote about how to create a bar graph using the <canvas> tag in html. This was more of a theory article, so as promised, here is the substance.

Start by creating your html body as follows:


<body onload="buildChart();">

  <div class="chart">

  <div id="label_container" style="position: relative;	margin: 20px auto; display: inline-block;">

  <canvas id="canvas" width="400" height="200"></canvas>

  </div>

  </div>

  <table id="mydata">

  <tr>       <th>Customers</th><th>Value</th> </tr>

  <tr><td>Design</td>  <td>2</td>  </tr>

  <tr><td>Enquiries</td>  <td>5</td>  </tr>

  <tr><td>Home Screen</td>  <td>1</td>  </tr>

  <tr><td>       Navigation</td>  <td> 4</td>  </tr>

  <tr><td>     Projects</td>  <td> 6</td>  </tr>

  <tr><td>    To-do</td>  <td> 9</td>  </tr>

  </table>

  </body>

The table contains the data used to build the chart, this can be built dynamically which is really useful for reports etc.

Now lets start on the javascript to build our chart. Firstly, lets start working with our buildChart function.


function buildChart(){
 // source data table and canvas tag

  var data_table = document.getElementById('mydata');

  var canvas = document.getElementById('canvas');

  var td_index = 1; // which TD contains the data

  var max_value = 0;

  var ySegmentCount = 5;

Here, we've defined our variables for the table, the canvas and specified the column that contains the values for our graph. max_value will be set to the largest of the values, and ySegmentCount contains the number of values to be listed horizontally.

I've defined a fixed pallet for this example, however you can easily adapt this to use a dynamic pallet, I just like to control the colours used.



  var myPallet = new Array();

  myPallet[0] = "#2A416A";

  myPallet[1] = "#39588E";

  myPallet[2] = "#4572AD";

  myPallet[3] = "#688BC3";

  myPallet[4] = "#8EA6D2";

  myPallet[5] = "#B2C3DF";

  myPallet[6] = "#CCD8EA";

  myPallet[7] = "#DDDBFB";

  myPallet[8] = "#F0F0F0";

  myPallet[9] = "#2A416A";

  myPallet[10] = "#39588E";

  myPallet[11] = "#4572AD";

  myPallet[12] = "#688BC3";

  myPallet[13] = "#8EA6D2";

  myPallet[14] = "#B2C3DF";

  myPallet[15] = "#CCD8EA";

  myPallet[16] = "#DDDBFB";

  myPallet[17] = "#F0F0F0";

Next we retrieve the chart values and calculate some parameters.


var tds, data = [], color, colors = [], value = 0;

  var trs = data_table.getElementsByTagName('tr'); // all TRs

  

  

  for (var i = 0; i < trs.length; i++) {

  tds = trs[i].getElementsByTagName('td'); // all TDs

  

  if (tds.length === 0) continue; //  no TDs here, move on

  

  // get the value, update total

  value  = parseFloat(tds[td_index].innerHTML);

  if(value > max_value){ max_value = value; } 

  data[data.length] = value;

  

  // Assign color

  color = myPallet[i];

  colors[colors.length] = color; // save for later

  trs[i].style.backgroundColor = color; // color this TR

  }

  var itemCount = trs.length-1;

  

  var yIncrement = max_value / ySegmentCount; //determine increment between y axis increments

  var barWidth = (canvas.width - (10 * (itemCount * 2))) / itemCount;

  var yIncrementPixels = (canvas.height - (2 * (ySegmentCount * 2))) / ySegmentCount;

We now know how wide to make the bars based on the canvas width and number of items, and also the distance between value markers.

We now need to prepare to draw our bar graph, we start by preparing the variables.


var ctx = canvas.getContext('2d');

var canvas_size = [canvas.width, canvas.height]; 



var backSoFar = 2;

var ySpanPosition = 0;

var yTotal = max_value;

backSoFar contains the vertical (y) start position of each background block. This starts at 2 to create a slight gap at the top. ySpanPosition contains the vertical position of the span containing the axis value.

Lets render the background.


for(i=0; i<ySegmentCount+1; i++){

  

if(i==ySegmentCount){



yTotal = canvas.height-8;



var labelContainer = document.getElementById('label_container');

var oSpan = document.createElement('span');

var oLabelText = document.createTextNode('0');

oSpan.setAttribute('style','left: -50px; top: '+yTotal+'px; position: absolute; font-size: 60%; z-index: 100; background: none; width: 45px; display: block; text-align: right;');

oSpan.appendChild(oLabelText);

labelContainer.insertBefore(oSpan, labelContainer.lastChild);

}else{ 

backSoFar += 2;

ySpanPosition = backSoFar -8;



ctx.fillStyle = '#E7EEF6';

ctx.fillRect(0,backSoFar,canvas.width,yIncrementPixels);



var labelContainer = document.getElementById('label_container');

var oSpan = document.createElement('span');

var oLabelText = document.createTextNode(yTotal.toFixed(2));

//var oLabelText = document.createTextNode('sdfsg');

oSpan.setAttribute('style','left: -50px; top: '+ySpanPosition+'px; position: absolute; font-size: 60%; z-index: 100; background: none; width: 45px; display: block; text-align: right;');

oSpan.appendChild(oLabelText);





labelContainer.insertBefore(oSpan, labelContainer.lastChild);

yTotal += -yIncrement;



backSoFar += yIncrementPixels;

backSoFar += 2;

} 

}

Change the value of ctx.fillStyle to set the background colour. Now for our final block for javascript in this function, we render the bars themselves.


var sofar = 0; // keep track of progress

// loop the data[]

for (var piece in data) {



var percentHeight = (data[piece]/max_value)*100;

var pixelHeight = (canvas.height-4)*(percentHeight/100);



var barFloor = canvas.height - pixelHeight; //subtract bar height from canvas height to get ground level;

sofar += 10; //add left margin



//ctx.beginPath();



ctx.fillStyle = colors[piece];    // color

ctx.lineWidth = 4;

ctx.strokeStyle = '#FFF'; 



ctx.shadowBlur       = 3;

ctx.shadowColor   = "#999";

ctx.shadowOffsetX = 0;

ctx.shadowOffsetY = 0;

ctx.strokeRect(sofar,barFloor,barWidth,pixelHeight);



ctx.shadowBlur       = 0;

ctx.shadowColor   = "#333";

ctx.fillRect(sofar,barFloor,barWidth,pixelHeight);



//Add labels

var xLabelPosition = sofar;

var yLabelPosition = canvas.height + 10; //add span height



var labelContainer = document.getElementById('label_container');

var oSpan = document.createElement('span');

var oLabelText = document.createTextNode(data[piece]);

//var oLabelText = document.createTextNode('sdfsg');

oSpan.setAttribute('style','left: '+xLabelPosition+'px; top: '+yLabelPosition+'px; position: absolute; font-size: 60%; z-index: 100; background: none; width: '+barWidth.toFixed(0)+'px; display: block; text-align: center;');

oSpan.appendChild(oLabelText);



labelContainer.insertBefore(oSpan, labelContainer.lastChild);





sofar += barWidth; 

sofar += 10; 

}

}

After calculating the positioning of the bar, we create 2 rectangles, the first is white and has drop shadow, the second is the coloured bar. Then we access the DOM to create the span labels and position them appropriately. The 10 value added to the sofar variable spaces the bars 10 pixels.

Finally, some css to make sure everything displays as required.


canvas { background: #fff; border-bottom: 1px solid #ccc; }

  .chart {

  position: relative;

  display: block;

  text-align: center;

  background: #FFF;

  padding-bottom: 10px;

  padding-top: 10px;

  }

  span { font-family: Arial, Helvetica, sans-serif;}

So there we have it, a low bandwidth bar graph that looks great and is totally dynamic. I haven't had time to make the code as efficient as possible but it definitely does the job. I hope you find this useful.

Tags: Javascript Tutorial HTML Canvas Charting Graphing Chart Tabulated Bar Graph Guide Low_Bandwidth Reporting Visualisation Data Table

Comments [0]

Add Comment

Navigation

Blog Categories