You may have noticed that on the chart above the center of the tickmarks are not transparent - they're just colored black in order to make them blend in with the rest of the chart.
Making them transparent is more complicated than you might think for these reasons:
globalCompositeOperation
option but with only one
canvas
it clears the background grid - so the line can't be seen and nor
can the grid. The result would be similar to the first situation
in this list.
The solution here is to use layers. "But wait! canvas
doesn't have layers!"
you might cry. And yes - you would be correct - but they can be simulated
by using - in this case - multiple canvas
tags.
So there are two canvas
tags being utilised here (positioned one in front of
the other) - one on to which the background is drawn and a second on to
which the lines are drawn. When the insides of the tickmarks are cleared
to transparency then the canvas
at the back shows through. And that
is the canvas
that has the background grid on it.
There's more code required than a regular chart (as you might imagine) - but as you can see it's not a lot more and nor is it complicated. Feel free to copy and paste the code.
<script src="RGraph.common.core.js"></script> <script src="RGraph.line.js"></script>Put this where you want the chart to show up:
<div style="position: relative; background-color: black; width: 750px; height: 250px"> <canvas id="cvs1" width="750" height="250" style="position: absolute; top: 0; left: 0">[No canvas support]</canvas> <canvas id="cvs2" width="750" height="250" style="position: absolute; top: 0; left: 0">[No canvas support]</canvas> </div>This is the code that generates the chart - it should be placed AFTER the
div
tag:
<script> line1 = new RGraph.Line({ id: 'cvs1', data: [null,null,null,null,null], options: { axes: false, backgroundGridColor: '#999', yaxisLabels: false, arksSize: 9 } }).draw(); var line2 = new RGraph.Line({ id: 'cvs2', data: [ [9,9,0,9,0], [6,7,10,5,0], [9,4,0,3,4] ], options: { colors: ['#cc0','blue','red'], axes: false, linewidth: 3, backgroundGrid: false, xaxisLabels: ['5.0','6.0','7.0','8.0','9.0'], textColor: '#ccc', tickmarksSize: 6, tickmarksStyle: function (obj, data, value, index, x, y, color, prevX, prevY) { // Draw the fill of the tickmark obj.path( 'gco destination-out b a % % % 0 6.29 false f red s % gco source-over', x, y, obj.get('tickmarksSize') - 1, color ); // This draws the outline of the tickmark obj.path( 'b lw 2 a % % % 0 6.29 false s %', x, y, obj.get('tickmarksSize'), color ); }, tickmarkSize: 6, responsive: [ {maxWidth:null,width:650,height:250,options:{textSize: 12,tickmarksSize:5},parentCss:{'float':'right'},callback:function (obj) {},callback:function (obj){var canvas = document.getElementById('cvs1');canvas.width = 650;canvas.height = 250;var div = canvas.parentNode.parentNode;div.style.width = '650px';div.style.height = '250px';RGraph.cache = [];RGraph.redraw();}}, {maxWidth:800,width:450,height:200,options:{textSize: 10,tickmarksSize:4},parentCss:{'float':'none'},callback:function (obj){var canvas = document.getElementById('cvs1');canvas.width = 450;canvas.height = 200;var div = canvas.parentNode.parentNode;div.style.width = '450px';div.style.height = '200px';RGraph.cache = [];RGraph.redraw();}} ] } }).draw(); </script>