async_chart_controller.js

import Chart from 'stimulus-chartjs'

// Connects to data-controller="async-chart"
export default class extends Chart {
  static values = {
    url: String
  }

  connect() {
    fetch(this.urlValue)
      .then(response => response.json())
      .then(data => {
        this.dataValue = data
        super.connect()
      })
  }
}