Vertical stack bar displays zeroes / “invisible” values with Chart.js

I’m trying to make a vertical stack bar plot with Chart.js.
It works pretty well but for some labels (i.e., for one vertical part of the graph) I don’t necessarily have strictly positive values for all the dataset points. I don’t see the rectangle for those which is good, but if I hover at the wrong place, I see a 0 which bothers me.

Here is a MRE because my description was probably not very understandable:

var chart = new Chart(ctx, {
   type: 'bar',
   data: {
      labels: ['1', '2'],
      datasets: [{
         label: 'a',
         data: [1, 8],
         backgroundColor: '#22aa99'
      }, {
         label: 'b',
         data: [1, 2],
         backgroundColor: '#109618'
      }, {
         label: 'c',
         data: [0, 1],
         backgroundColor: '#dc3912'
      }, {
         label: 'd',
         data: [4, null],
         backgroundColor: '#3366cc'
      },
      {
         label: 'e',
         data: [4, null],
         backgroundColor: '#9661ca'
      }]
   },
   options: {
      responsive: false,
      legend: {
         position: 'right'
      },
      scales: {
         xAxes: [{
            stacked: true
         }],
         yAxes: [{
            stacked: true
         }]
      }
   }
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.6.0/Chart.min.js"></script>
<canvas id="ctx" width="700"></canvas>

I would like this not to be possible (hovering on d on the right part even though d is 0 (as well as e)):
enter image description here

Using data: [4, null] does not work either, since I have that now instead:
enter image description here