Nithya.K Nithya.K - 3 months ago 149
Javascript Question

How to display pie chart data values of each slice in chart.js

I am using Chart.js for drawing pie chart in my php page.I found tooltip as showing each slice values.

enter image description here

But I wish to display those values like below image. enter image description here

I do not know how to do this with chart.js.

Please help me.

My Javascript code:

function drawPie(canvasId,data,legend){
var ctx = $("#pie-canvas-" + canvasId).get(0).getContext("2d");

var piedata = [];
$.each(data,function(i,val){
piedata.push({value:val.count,color:val.color,label:val.status});
});
var options =
{
tooltipTemplate: "<%= Math.round(circumference / 6.283 * 100) %>%",
}
var pie = new Chart(ctx).Pie(piedata,options);
if(legend)document.getElementById("legend").innerHTML = pie.generateLegend();
}


php code:

printf('<table><tr>');
echo '<td style="text-align: right;"><canvas id="pie-canvas-'
. $canvasId
. '" width="256" height="256" ></canvas></td><td style="text-align: left;width:360px;height:auto" id="legend" class="chart-legend"></td></tr></table>';

echo '<script type="text/javascript">drawPie('
. $canvasId
. ', '
. $data3
.', '
. $legend
. ');</script>';


enter image description here

Answer

From what I know I don't believe that Chart.JS has any functionality to help for drawing text on a pie chart. But that doesn't mean you can't do it yourself in native JavaScript. I will give you an example on how to do that, below is the code for drawing text for each segment in the pie chart:

function drawSegmentValues()
{
    for(var i=0; i<myPieChart.segments.length; i++) 
    {
        // Default properties for text (size is scaled)
        ctx.fillStyle="white";
        var textSize = canvas.width/10;
        ctx.font= textSize+"px Verdana";

        // Get needed variables
        var value = myPieChart.segments[i].value;
        var startAngle = myPieChart.segments[i].startAngle;
        var endAngle = myPieChart.segments[i].endAngle;
        var middleAngle = startAngle + ((endAngle - startAngle)/2);

        // Compute text location
        var posX = (radius/2) * Math.cos(middleAngle) + midX;
        var posY = (radius/2) * Math.sin(middleAngle) + midY;

        // Text offside to middle of text
        var w_offset = ctx.measureText(value).width/2;
        var h_offset = textSize/4;

        ctx.fillText(value, posX - w_offset, posY + h_offset);
    }
}

A Pie Chart has an array of segments stored in PieChart.segments, we can look at the startAngle and endAngle of these segments to determine the angle in between where the text would be middleAngle. Then we would move in that direction by Radius/2 to be in the middle point of the chart in radians.

In the example above some other clean-up operations are done, due to the position of text drawn in fillText() being the top right corner, we need to get some offset values to correct for that. And finally textSize is determined based on the size of the chart itself, the larger the chart the larger the text.

Fiddle Example


With some slight modification you can change the discrete number values for a dataset into the percentile numbers in a graph. To do this get the total value of the items in your dataset, call this totalValue. Then on each segment you can find the percent by doing:

Math.round(myPieChart.segments[i].value/totalValue*100)+'%';

The section here myPieChart.segments[i].value/totalValue is what calculates the percent that the segment takes up in the chart. For example if the current segment had a value of 50 and the totalValue was 200. Then the percent that the segment took up would be: 50/200 => 0.25. The rest is to make this look nice. 0.25*100 => 25, then we add a % at the end. For whole number percent tiles I rounded to the nearest integer, although can can lead to problems with accuracy. If we need more accuracy you can use .toFixed(n) to save decimal places. For example we could do this to save a single decimal place when needed:

var value = myPieChart.segments[i].value/totalValue*100;
if(Math.round(value) !== value)
    value = (myPieChart.segments[i].value/totalValue*100).toFixed(1);
value = value + '%';

Fiddle Example of percentile with decimals

Fiddle Example of percentile with integers

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