If you pass the JSON in your post to json_decode, it will fail. Valid JSON strings have quoted keys:

json_decode('{foo:"bar"}');         // this fails
json_decode('{"foo":"bar"}', true); // returns array("foo" => "bar")
json_decode('{"foo":"bar"}');       // returns an object, not an array.

var json_data = {"2013-01-21":1,"2013-01-22":7};
var result = [];

for(var i in json_data)
    result.push([i, json_data [i]]);


var data = new google.visualization.DataTable();
data.addColumn('string', 'Topping');
data.addColumn('number', 'Slices');
data.addRows(result);

http://jsfiddle.net/MV5rj/