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You need to extend chart.js typings by creating your own index.d.ts
file and put the following in it:
import {ChartType, Plugin} from 'chart.js';
declare module 'chart.js' {
interface PluginOptionsByType<TType extends ChartType> {
horizontalArbitraryLine?: {
lineColor: string
};
}
}
Source:stackexchange.com