This heatmap represents the position of a mobile device weighted by the duration of the stay. See the heatmap.js source for details.
The heatmap is a standard OpenLayers.Layer.Vector layer with the renderers option set to ['Heatmap'].
The features must have a style.weight value (between 0.0 and 1.0); this value can be extracted from the attributes.
Some code taken from https://github.com/substack/node-heatmap