Chart.js line chart set background color

Question

I\'m working with Chart.js and want to convert a line chart to a PNG. The problem is that the image always downloads with a transparent background, which is not what I need.

Answer 1

Here's one way to get a fully-opaque version of your ChartJS:

Wait until the chart is fully animated out and complete. You can do this by adding the onAnimationComplete property to the chart.

In the onAnimationComplete function:

• Create an in-memory temporary canvas of equal size as your chart.
• Fill the temp canvas with white
• drawImage the ChartJS canvas over the white-filled temp canvas
• Create an image from the temp canvas.

Here's how that might be done:

var ctx = document.getElementById("canvas").getContext("2d");
window.myLine = new Chart(ctx).Line(lineChartData, {
  responsive: true,
  onAnimationComplete:function(){
      var tcanvas=document.createElement('canvas');
      var tctx=tcanvas.getContext('2d');
      tcanvas.width=ctx.canvas.width;
      tcanvas.height=ctx.canvas.height;
      tctx.fillStyle='white';
      tctx.fillRect(0,0,tcanvas.width,tcanvas.height);
      tctx.drawImage(canvas,0,0);
      var img=new Image();
      img.onload=function(){
          document.body.appendChild(img);
      }
      img.src=tcanvas.toDataURL();
  }
});

Here's example code and a Demo:

var randomScalingFactor = function(){ return Math.round(Math.random()*100)};

var randomColorFactor = function(){ return Math.round(Math.random()*255)};

var lineChartData = {
  labels : ["January","February","March","April","May","June","July"],
  datasets : [
    {
      label: "My First dataset",
      fillColor : "rgba(220,220,220,0.2)",
      strokeColor : "rgba(220,220,220,1)",
      pointColor : "rgba(220,220,220,1)",
      pointStrokeColor : "#fff",
      pointHighlightFill : "#fff",
      pointHighlightStroke : "rgba(220,220,220,1)",
      data : [randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor()]
    },
    {
      label: "My Second dataset",
      fillColor : "rgba(151,187,205,0.2)",
      strokeColor : "rgba(151,187,205,1)",
      pointColor : "rgba(151,187,205,1)",
      pointStrokeColor : "#fff",
      pointHighlightFill : "#fff",
      pointHighlightStroke : "rgba(151,187,205,1)",
      data : [randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor()]
    }
  ]

}

var ctx = document.getElementById("canvas").getContext("2d");
window.myLine = new Chart(ctx).Line(lineChartData, {
  responsive: true,
  onAnimationComplete:function(){
    var tcanvas=document.createElement('canvas');
    var tctx=tcanvas.getContext('2d');
    tcanvas.width=ctx.canvas.width;
    tcanvas.height=ctx.canvas.height;
    tctx.fillStyle='white';
    tctx.fillRect(0,0,tcanvas.width,tcanvas.height);
    tctx.drawImage(canvas,0,0);
    var img=new Image();
    img.onload=function(){
      document.body.appendChild(img);
    }
    img.src=tcanvas.toDataURL();
  }
});

ChartJS line chart

  

    
  

Fully opaque chart as image