Normally, the chart titles are centered over plot area. If you need them centered over the whole chart area, you can use labels instead.
For that, use allLabels
chart setting.
You may also need to use marginTop
in order to give the label/title some space.
"allLabels": [{ "text": "This is chart-area-centered label", "align": "center", "bold": true, "size": 20, "y": 10 }], "marginTop": 50