heatmap.js is a JavaScript library that can be used to generate web heatmaps with the html5canvas element based on your data. 
Heatmap instances contain a store in order to colorize the heatmap based on relative data, which means if you're adding only a single datapoint to the store it will be displayed as the hottest(red) spot, then adding another point with a higher count, it will dynamically recalculate. 
The heatmaps are fully customizable - your welcome to choose your own color gradient, change it's opacity, datapoint radius and many more. 
heatmap.js is dual-licensed under the MIT and the Beerware license, feel free to use it in your projects.