Join JSON objects on common property
I notice a query on one of your urls. I suggest you use jQuery's $.param to construct the query cleanly.

For your first loop, you could use reduce instead of forEach to generate your hash. Additionally, you might want kill NaN early. You don't want to be carrying NaN in any data structure, otherwise you'll get some unexpected NaN in your code.

var dataHash = data.reduce(function(hash, item) {
  var value = +item.value
  hash[item.label] = isNaN(value) ? null : value;
  return hash;
}, {});

geojson.features.forEach(function(item) {
  item.properties.incidents = dataHash[item.properties._feature_id];
});


As for efficiency, I guess that's about it. You don't want to be nesting a lookup inside the other loop. Two separate loops is better then one nested inside another.