I probably found the solution.
At first $.jqplot have to be inside $.getJSON - I forgot about asynchronous invocation code in JavaScript.
I unnecessarily added quote mark to data[index].data
line.push(["'"+data[index].data+"'",data[index].kurs_odn]);
But I had to add Number(data[index].kurs_odn) becouse that was string by default.
Now it seems working fine.