Eliminating duplicate labels in a tooltip in a ChartVoronoiComponent?

[rhwood]

I am using typescript, but am getting a compilation error when attempting to implement the example code for labels in the Scatter+Line chart at https://www.patternfly.org/charts/scatter-chart#line-chart

The example shows that labels property can be defined as

<ChartVoronoiContainer
    labels={({ datum }) => datum.childName.includes('line-') ? `${datum.name}: ${datum.y}` : null}
    constrainToVisibleArea
/>

and the error is:

Type '({ datum }: any) => string | null' is not assignable to type '(point: any, index: number, points: any[]) => string'.
  Type 'string | null' is not assignable to type 'string'.
    Type 'null' is not assignable to type 'string'. ts(2322)
ChartVoronoiContainer.d.ts(93, 5): The expected type comes from property 'labels' which is declared here on type 'IntrinsicAttributes & ChartVoronoiContainerProps

Am I doing something wrong or is this a bug in how ChartVoronoiComponent is defined that works in JavaScript, but not in TypeScript?