Jelch,
Here y values are string. So, by parsing newly assigned y values to number will render the updated chart. Change the click handler function as below-
$("#AddDate").click(function () {
dataPointsK.push({ x:TrainingNr ,y: Number(document.getElementById("deadlift").value)});
dataPointsB.push({ x: TrainingNr,y: Number(document.getElementById("benchpress").value)});
TrainingNr = TrainingNr +1
chart.render();
});