Custom tooltip formatting is configured through the tooltip object in the chart options:
headerFormat Type: String | when set to null will disable the header.
pointFormat Type: String | defines the HTML template for each data point and supports custom HTML when useHTML is enabled.
useHTML Type: boolean (default false) | enables HTML rendering in tooltips.
{point.name} and {point.y} are useful values that can be referenced for each point in the graph.