Two weeks?
gradient: {
0 : '#00f',
0.2 : '#00f',
0.2001: '#0ff',
0.4 : '#0ff',
0.4001: '#0f0',
0.6 : '#0f0',
0.6001: '#ff0',
0.8 : '#ff0',
0.8001: '#f00',
1 : '#f00'
}
I haven't tested this, but then again I have never used heatmap.js. I just looked at the docs for thirty seconds, and I figure this should work.