title: Chartjs year markings
body: <p>When I make a Chartjs table the background is a plain square pattern. What I would like is to be able to mark the transition from one year to the next with a bolder vertical line at every new year.</p>
  <p>In the below example chart, I have 12 quarters. I would like to have a line appear between the Q1s and Q4s. Is this possible?</p>
  <pre class="lang-js prettyprint-override"><code>chart_code = new Chart(ctx, {
  type: 'line',
  data: {
    labels: ['Q1 15','Q2 15','Q3 15','Q4 15','Q1 16','Q2 16','Q3 16','Q4 16','Q1 17','Q2 17','Q3 17','Q4 17'],
    datasets: [{
      label: 'Teststatistic',
      data: [50,40,30,20,50,20,10,10,10,40,30,80],
      backgroundColor: ['rgba(0, 105, 146, 1)'],
      borderColor: ['rgba(0, 105, 146, 1)'],
      borderWidth: 3,
      fill: false,
      pointStyle: 'rect',
      pointRadius: 5
    }]
  },
  options: {
    responsive: false,
    plugins: {
      legend: {
        display: false,
        boxWidth: 80,
        boxHeight: 80
      }
    },
    scales: {
      x: { 
        ticks: {
          maxRotation: 0, 
          minRotation: 0
        } 
      }
    }
  }
  </code></pre>