1 d3.xhr = function(url, mime, callback) {

     2   var req = new XMLHttpRequest;

     3   if (arguments.length < 3) callback = mime;

     4   else if (mime && req.overrideMimeType) req.overrideMimeType(mime);

     5   req.open("GET", url, true);

     6   req.onreadystatechange = function() {

     7     if (req.readyState === 4) callback(req.status < 300 ? req : null);

     8   };

     9   req.send(null);

    10 };