title: ChartJS add tooltip to a grouped bar chart
body: <p>Iam trying to add an euro sign to the tooltips of my grouped bar chart using ChartJS. Snipped:</p>
  
  <pre><code>tooltips: {
     mode: 'label',
     callbacks: {
        label: function(tooltipItem, data) {
           return data['datasets'][0]['data'][tooltipItem['index']] + '€';
        }
     }
  }
  </code></pre>
  
  <p>This code works for my linechart, but not for my grouped bar chart. I want my bar chart to look like the following, when I hover it:</p>
  
  <p><a href="https://i.stack.imgur.com/mOIMc.png" rel="noreferrer"><img src="https://i.stack.imgur.com/mOIMc.png" alt="enter image description here"></a></p>
  
  <p>But there is no euro sign in my chart, it just display its value. What am I doing wrong?</p>
  
  <p>Thank you.</p>
  
  <p>** Edit</p>
  
  <p>So my full options looked like the following:</p>
  
  <pre><code>options: {
              title: {
                  display: true,
                  text: 'Title',
              },
              scales: {
                  yAxes: [{
                      scaleLabel: {
                          display: true,
                          labelString: 'Wert in €'
                      }
                  }],
                  xAxes: [{
                      scaleLabel: {
                          display: true,
                          labelString: 'Zeitintervall'
                      }
                  }]
              },
              tooltips: {
                  mode: 'label',
                      callbacks: {
                          label: function(tooltipItem, data) {
                              return data['datasets'][0]['data'][tooltipItem['index']] + '€';
                          }
                      }
              }
          }
  </code></pre>
  
  <p>As soon as i removed the <code>scales</code>, it is showing the euro sign.
  So my options now look like the following:</p>
  
  <pre><code>options: {
              title: {
                  display: true,
                  text: 'Title'
              },
              tooltips: {
                  mode: 'label',
                  callbacks: {
                      label: function(tooltipItem, data) {
                          return data['datasets'][0]['data'][tooltipItem['index']] + ' €';
                      }
                  }
              }
          }
  </code></pre>
  
  <p>But now i got another problem, it shows the same value for two different bars:
  <a href="https://i.stack.imgur.com/cE11o.png" rel="noreferrer"><img src="https://i.stack.imgur.com/cE11o.png" alt="enter image description here"></a></p>
  
  <p>You can see clearly that the values are not the same. Whats the problem here?</p>