An SVG Line with a simple look

• How to get transparent tickmarks using the canvas Line chart

Here is a simple Line chart that has three separate datasets. The labels for the chart (on the x-axis) are not a scale - they're just regular labels as you would normally give to a Line chart but they're numbers instead of words - that's all. The Line doesn't have the facility to show a scale across the x-axis - for that you'll need to use a Scatter chart with the line option enabled.

As for the tickmarks, while at first glance it may appear that they're 'hollow' they're actually just using a fill color that matches the background.

Other than this it's a pretty basic dark-themed Line that uses the trace effect.

This goes in the documents header:

<script src="RGraph.svg.common.core.js"></script>
<script src="RGraph.svg.line.js"></script>

Put this where you want the chart to show up:

<div style="float: right">
    <div id="cc" style="width:700px; height: 300px; background-color: black"></div>
</div>

This is the code that generates the chart - it should be placed AFTER the div tag:

<script>
    line = new RGraph.SVG.Line({
        id: 'cc',
        data: [
            [9,9,0,9,0],
            [6,7,10,5,0],
            [9,4,0,3,4]
        ],
        options: {
            colors: ['#cc0','blue','red'],
            xaxis: false,
            yaxis: false,
            linewidth: 3,
            backgroundGridColor: '#999',
            xaxisLabels: ['5.0','6.0','7.0','8.0','9.0'],
            textColor: '#ccc',
            tickmarksStyle: 'circle',
            tickmarksFill: 'black',
            tickmarksLinewidth: 3,
            responsive: [
                {maxWidth:null,width:700,height:300,options:{textSize: 12,tickmarksSize:5},css:{'float':'right'}},
                {maxWidth:800,width:450,height:200,options:{textSize: 10,tickmarksSize:4},css:{'float':'none'}}
            ]
        }
    }).trace();
</script>

How to get transparent tickmarks using the canvas Line chart

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:

• If you use a transparent fill then the Line itself would still be visible.
• If you use a black fill it overwrites the background grid (this is the first chart).
• If you use the 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.

This goes in the documents header:

<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: 650px; 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: {
            xaxis: false,
            yaxis: false,
            backgroundGridColor: '#999',
            yaxisLabels: false
        }
    }).on('beforedraw', function (obj)
    {
        RGraph.clear(obj.canvas, 'black');
    }).draw();


    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'],
            xaxis: false,
            yaxis: 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){var canvas = document.getElementById('cvs1');canvas.width  = 650;canvas.height = 250;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;RGraph.cache = [];RGraph.redraw();}}
            ]
        }
    }).draw();
</script>