Canvas sparklines

I like sparklines a lot. Tufte describes a sparkline as:

…a small intense, simple, word-sized graphic with typographic resolution.

Four years ago, I added sparklines to Huffduffer using Google’s chart API. That API comes in two flavours: a JavaScript API for client-side creation of graphs, and image charts for server-side rendering of charts as PNGs.

The image API is really useful: there’s no reliance on JavaScript, it works in every browser capable of displaying images, and it’s really flexible and customisable. Therefore it is, of course, being deprecated.

The death warrant for Google image charts sets the execution date for 2015. Time to start looking for an alternative.

I couldn’t find a direct equivalent to the functionality that Google provides i.e. generating the images dynamically on the server. There are, however, plenty of client-side alternatives, many of them using canvas.

Most of the implementations I found were a little heavy-handed for my taste: they either required jQuery or Processing or both. I just wanted a quick little script for generating sparklines from a dataset of numbers. So I wrote my own.

I’ve put my code up on Github as Canvas Sparkline.

Here’s the JavaScript. You create a canvas element with the dimensions you want for the sparkline, then pass the ID of that element (along with your dataset) into the sparkline function:

sparkline ('canvasID', [12, 18, 13, 12, 11, 15, 17, 20, 15, 12, 8, 7, 9, 11], true);

(that final Boolean value at the end just indicates whether you want a red dot at the end of the sparkline).

The script takes care of normalising the values, so it doesn’t matter how many numbers are in the dataset or whether the range of the numbers is in the tens, hundreds, thousands, or hundreds of thousands.

There’s plenty of room for improvement:

  • The colour of the sparkline is hardcoded (50% transparent black) but it could be passed in as a value.
  • All the values should probably be passed in as an array of options rather than individual parameters.

Feel free to fork, adapt, and improve.

The sparklines are working quite nicely, but I can’t help but feel that this isn’t the right tool for the job. Ideally, I’d like to keep using a server-side solution like Google’s image charts. But if I am going to use a client-side solution, I’m not sure that canvas is the right element. This should really be SVG: canvas is great for dynamic images and animations that need to update quite quickly, but sparklines are generally pretty static. If anyone fancies making a lightweight SVG solution for sparklines, that would be lovely.

In the meantime, you can see Canvas Sparkline in action on the member profiles at The Session, like here, here, here, or here.

Update: Ask and thou shalt receive. Check out this fantastic lightweight SVG solution from Stuart—bloody brilliant!

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