混合图表(Mixed Chart Types)

With Chart.js, it is possible to create mixed charts that are a combination of two or more different chart types. A common example is a bar chart that also includes a line dataset.

When creating a mixed chart, we specify the chart type on each dataset.

```javascript
const mixedChart = new Chart(ctx, {
    data: {
        datasets: [{
            type: 'bar',
            label: 'Bar Dataset',
            data: [10, 20, 30, 40]
        }, {
            type: 'line',
            label: 'Line Dataset',
            data: [50, 50, 50, 50],
        }],
        labels: ['January', 'February', 'March', 'April']
    },
    options: options
});
```

At this point, we have a chart rendering how we'd like. It's important to note that the default options for the charts are only considered at the dataset level and are not merged at the chart level in this case.

## Drawing order
By default, datasets are drawn such that the first one is top-most. This can be altered by specifying order option to datasets. order defaults to 0. Note that this also affects stacking, legend, and tooltip. So it's essentially the same as reordering the datasets.

The order property behaves like a weight instead of a specific order, so the higher the number, the sooner that dataset is drawn on the canvas and thus other datasets with a lower order number will get drawn over it.

```javascript
const mixedChart = new Chart(ctx, {
   type: 'bar',
   data: {
       datasets: [{
           label: 'Bar Dataset',
           data: [10, 20, 30, 40],
           // this dataset is drawn below
           order: 2
       }, {
           label: 'Line Dataset',
           data: [10, 10, 10, 10],
           type: 'line',
           // this dataset is drawn on top
           order: 1
       }],
       labels: ['January', 'February', 'March', 'April']
   },
   options: options
});
```