Q:

Get value of json string

[
    {
        "type": "spline",
        "name": "W dor\u0119czeniu",
        "color": "rgba(128,179,236,1)",
        "mystring": 599,
        "data": ...
    }
]

I am trying to access this json as json['W doręczeniu']['mysting'], and I get no value why is that? 

A:

The value of "W doręczeniu" is not a key, so you cannot use it to get a value.  Since your json string is an array you'll have to do json[0].nameto access the first (and only) element in the array, which happens to be the object.  Of course, this is assuming json is the variable you store the array into. 
var json = [{"type":"spline","name":"W dor\u0119czeniu","color":"rgba(128,179,236,1)","mystring":599}];

console.log(json[0].mystring); //should give you what you want. 

EDIT:
To get the last element in a js array, you can simply do this:
console.log( json[json.length -1].mystring ); // same output as the previous example

'length - 1' because js arrays are indexed at 0.  There's probably a million and one ways to dynamically get the array element you want, which are out of the scope of this question.