Horizon charts

I have been slowly working on a metrics collection and monitoring service. There are many others, but I wanted something very simple to feed and to integrate with a wiki to monitor/display. During the development of the service I discovered Horizon charts by way of (the brilliant) Cubism.js. The goal of an horizon chart is to use the a minimum of vertical height without loss of precision. Horizon charts look like bar charts. However, horizon charts use both the top and bottom edge as axises. The top edge is used to show values below a threshold with the bars going downward and the bottom edge is used to show values above the threshold with the bars going upward. Further, to extend the range of a value beyond the height of the chart the values are "folded" and the folds layered on the chart. Each fold is drawn as a bar on top of the previous fold's bar. The illustration at Sizing the Horizon: The Effects of Chart Size and Layering on the Graphical Perception of Time Series Visualizations shows this very well: I was a little intimidated by Cubism.js and D3.js; they are sophisticated toolkits that will take more time for me to understand than I wanted to commit just now. Plus I really wanted to learn more about using HTML's canvas element, and so I set about my own implementation. The code at https://gist.github.com/4264347 is my first cut at the implementation. It works for a fixed, two fold horizon chart. The example chart below plot the values between +/-200 with folds at +/-100.

Update: Updated the working example to be more general. Removed the code from this posting in preference to the Gist. FYI: To run the code just download the gist and open it in a browser. To play with it copy the gist into the HTML panel of https://jsfiddle.net/ and run.