average the last n samples and emit the result as a stream
var avg = require('avg-stream');
var s = avg(2);
s.on('data',function(d){
console.log(d);
})
s.write({a:1,b:2})
s.write({a:3,b:1})
s.write({a:2,b:4})
output
{ a: 1, b: 2 }
{ a: 2, b: 1.5 }
{ a: 2.5, b: 2.5 }