- About the RGraph software
- About the author
- About the SVG and canvas tags
- History of the tags
- About HTML5 charts
- About canvas text
- Contact information and support
About the RGraph software
RGraph is 100% unrestricted to use, being available under the MIT license. You don't even have to send me a Christmas card! Any donations are appreciated - (you can find out more on the donations page).
About the author
About the SVG and canvas tags
SVG, on the other hand, has a DOM just like the HTML document and each element (lines, circles rectangles etc) can be referenced directly like HTML elements can in your document.
The <canvas> tag uses a "fire and forget" drawing methodology - there is no DOM that's maintained, so if you want to alter something you'll probably (but not necessarily) have to redraw the entire canvas.
The lack of a DOM means that canvas is fast to draw on and very responsive - important when you're providing interactive or animated charts to your users.
SVG uses a drawing methodology that is similar to your HTML page - where each element is an object in a DOM that can be referenced.
When you update the properties of these objects the scene is converted to a bitmap and displayed automatically for you.
Other uses for the <svg> and
<canvas> include providing a control panel to your
users and using it to create games. You should note though that when it
comes to accessibility then a more traditional HTML-based interface that
<canvas> for certain elements and SVG for others may be
History of the tags
The canvas tag was originally introduced by Apple in 2004 for use in Mac OS X WebKit to power dashboard applications and their Safari web browser.
Since then it has been adopted by Mozilla and Opera and now the W3C has adopted it in the upcoming HTML5 specification. It's now supported by all modern web browsers including modern versions of MSIE (starting from version 9).
The SVG tag is a bit like canvas but instead of being a bitmap drawing surface it's vector-based and has a structure more like an HTML document.
SVG is XML-based, an open standard and has been in development by the W3C since 1999. All modern browsers have some degree of support for SVG.
It's currently at version 1.1 with version 2 on its way which will use CSS integration more.
Here's an example of both tags:
<canvas width="200" height="120" id="cvs" style="border: 1px solid gray"></canvas> <script> // Get hold of references to the canvas tag and the 2D drawing context canvas = document.getElementById('cvs'); context = canvas.getContext('2d'); // Draw the red square context.beginPath(); context.rect(10,10,50,50); context.fillStyle = 'red'; context.fill(); // Draw the blue circle context.beginPath(); context.arc(130,60,50,0, 2 * Math.PI * 2, false); context.fillStyle = 'blue'; context.fill(); </script>
<svg width="200" height="120" version="1.1"> <!-- The red rectangle --> <rect x="10" y="10" width="50" height="50" fill="red"></rect> <-- The blue circle --> <circle cx="130" cy="60" r="50" fill="blue"></circle> </svg>
The output of the SVG example is more or less the same as the canvas version but the difference comes in how the two are created. As you can see the SVG code required is far smaller even in this small example.
About HTML5 charts
What makes the canvas and SVG tags good for producing charts is the ability to interact seamlessly with both the user and the rest of the page.
Modern browsers that support HTML5 also support canvas and SVG, including Internet Explorer 9 and upwards (with version 9 you may need to use the HTML5 doctype, but not from version 10).
About canvas text
The canvas tag, despite being very versatile, does not render text very well. As far as the quality is concerned it gets worse at higher zoom levels. It's also not "real" text - so you can't select it or copy it to your clipboard and paste it elsewhere.
So for this reason RGraph has an accessible text option. What this does is wrap the canvas in a div tag and use a combination of relative and absolute positioning to position <span> tags over the canvas that contain the relevant text.
It's not perfect for every situation and there's a list (that's not comprehensive) of caveats on that page.
It does make text look much better though, and as less is being drawn onto the canvas it can make a noticeable performance improvement - particularly in canvas-based effects and animations.
Thanks go to:
- Erwan Al for the nudge to create the HTMLTable reader
- Joachim Schmidt for suggesting the Line chart
- Mike Robinson for a fix to the canvas Rose chart
- Brian Tozer for lots and lots of SEO help
- Claus Tondering for catching a bug in the event system.
- Graeme Elsworthy for some updates and documentation to the Gauge chart labels.
- Anthony Kuma for some updates to the Line chart
- Zsolt for some updates to the Line chart documentation and Horizontal Bar chart updates.
- Amjad Moghul for the documentation layout style.
- Jan Dirk for various bug reports.
Contact information and support
RGraph on Wikipedia
RGraph has a Wikipedia page! You can read about RGraph and its history on Wikipedia as well as on this page.