RGraph is a JavaScript charts library based on HTML5 SVG and canvas. RGraph is mature (over 15 years old) and has a wealth of features making it an ideal choice to show charts on your website.

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Get the latest version of RGraph (version 6.17) from the download page. There's also older versions available, minified files and links to cdnjs.com hosted libraries.

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RGraph can be used for free under the GPL or if that doesn't suit your situation there's an inexpensive (£129) commercial license available.

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The getContext function

The getContext function is the function that you use to get access to the canvas tags 2D drawing functions. As of April 2014, there are two types of context that are available to you: 2d and webgl. These provide you with the api that you use to draw on the canvas. It should be noted that the webgl context is significantly more complicated than the 2d version.

A more recent addition is the ability to give a second argument of options (an object) - of which currently the only available option is the alpha setting. By setting this you're saying that the canvas has no transparency and the background is initialised to black. You can still use semi-transparent colors on top of that - but keep in mind that the background is now black.

Arguments to the function

An example

    window.onload = function ()
        var canvas  = document.getElementById("cvs");
        var context = canvas.getContext('2d');

        // Fetch the context and stipulate it as having no alpha/opacity
        // var context = canvas.getContext('2d', {alpha: false} );
        // var context = canvas.getContext('webgl');              // WebGL 3D ontext
        // var context = canvas.getContext('experimental-webgl'); // Older WebGL 3D context