Thanks Manoj! That worked.
Important to note (what probably is clear for the most people but was not for me): if you define the function in the windows.onload function you need to define your chart as global variable (windows.myChart = new ChanvasJs.Chart()” and access it via windows.myChart. Before you see the title change you have to rerender the plot “windows.myChart.render()”.
Cheers, Tschounes