Pie, bar, line: SVG/VML better than Canvas

Question

I need to choose a library for "standard" charting: pies, lines and bars.

From what I've read, it seems to me that the best format is SVG/VML, like Highcharts for example. SVG is becoming standard across all major browsers, now that IE 9 accepts it. It seems easier to rescale and export than Canvas.

Still, I see that several charting libraries rely on Canvas. Am I missing something? Is there any reason for considering Canvas over SVG for such applications?

Answer 1

You can generally achieve the same results with either. Both end up drawing pixels to the screen for the user. The major differentiators are that HTML5 Canvas gives you pixel-level control over the results (both reading and writing), while SVG is a retained-mode graphics API that makes it very easy to handle events or manipulate the artwork with JavaScript or SMIL Animation and have all redrawing taken care of for you.

In general, I'd suggest using HTML5 Canvas if you:

• need pixel-level control over effects (e.g. blurring or blending)
• have a very large number of data points that will be presented once (and perhaps panned), but are otherwise static

Use SVG if you:

• want complex objects drawn on the screen to be associated with events (e.g. move over a data point to see a tooltip)
• want the result to print well at high resolution
• need to animate the shapes of various graph parts independently
• will be including text in your output that you want to be indexed by search engines
• want to use XML and/or XSLT to produce the output

Answer 2

Canvas isn't needed unless you want heavy manipulation/animation or are going to have 10,000+ charts. More on performance analysis here.

It is also important to make the distinction: Charting and diagramming are two different things. Displaying a few bar charts is very different from (for instance) making diagramming flowcharts with 10,000+ movable, link-able, potentially-animated objects.

Every SVG element is a DOM element, and adding 10,000 or 100,000 nodes to the DOM causes incredible slowdown. But adding that many elements to Canvas is entirely doable, and can be quite fast.

In case it may have confused you: RaphaelJS (in my opinion the best charting SVG Library) makes use of the word canvas, but that is no way related to the HTML <canvas> element.