title: chartjs-plugin-annotations not displayed in angular 5
body: <p>While using the chart.js and the plugin chartjs-plugin-annotation, annotations are not showing while using angular 5, no error messages are displayed.</p>
  
  <p>I have created a cut down example of code that exhibits the problem</p>
  
  <p>console.log(Chart.plugins) shows the plugin looks to be registered as plugin[3] however it doesn't have an id as the inbuilt ones do, is this a problem?</p>
  
  <p><img src="https://user-images.githubusercontent.com/11596738/43621043-d43fdf0e-9718-11e8-8011-72c5ef12dce1.png" alt="image"></p>
  
  <p>chart.component.ts</p>
  
  <pre><code>import { Component, Inject } from '@angular/core';
  import { Chart } from 'chart.js';
  import 'chartjs-plugin-annotation';
  
  @Component({
    selector: 'app-chart-component',
    templateUrl: './chart.component.html'
  })
  export class ChartComponent {
    public currentCount = 0;
  
    chart : Chart ; // This will hold our chart info
  
    simpleChart() {
  
      console.log(Chart.plugins);
  
      this.chart = new Chart('canvas', {
        type: 'line',
        data: {
          labels: ['0','1','2', '3','4'],
          datasets: [
            {
              data: [0,1,2,5,4,5],
              borderColor: "#3cba9f",
              fill: false,
            },
  
          ]
        },
        options: {
          legend: {
            display: false
          },
          scales: {
            xAxes: [{
              display: true
            }],
            yAxes: [{
              display: true,
              id: 'y-axis-0'
            },
            ]
          },
          plugins: {
            annotation: {
              annotations: [{
                type: 'line',
                id: 'hLine',
                mode: 'horizontal',
                scaleID: 'y-axis-0',
                value: 2.5,  // data-value at which the line is drawn
                borderWidth: 2.5,
                borderColor: 'black'
              }]
            }
          }
        }
      });
    }
  
    ngOnInit() {
      this.simpleChart();
    }
  }
  </code></pre>
  
  <p>Any assistance would be greatly appreciated.</p>