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Well the reactivePropMixin will not work in your example. Because it will only work if you pass the whole chart.js data object as a prop. Not only the data array for one dataset. It is quite limited. However you can simply add an own watcher and then call this.$data._chart.update()

The main problem with API requests is that they are async. So you component gets mounted, request the data and your chart gets rendered with no data, because your api call is maybe not finished at the time. Then it finishes, mutate your data and your chart will break. To prevent this it is a common practice to add a condition. In your promise or async call you need to define something like isLoaded and if you finished the api call you can set it to true. And then add a condition to your chart v-if="isLoaded" so your chart gets initially only rendered if you have your data from the API.