[FEATURE] Datasets as a map/object instead of (just) arrays

I have data I'm adding dynamically to a chart where some data sources have more things to plot than others

So ideally I would do
```
mchart.data.datasets[key].data[i]
```

where key is not a number - that way I don't have to worry about preprocessing the data an ensuring I have a mapping from key -> chart.js index myself

I realize the tradeoff is then there is no set order (maybe alphabetical order?) in the display of the legend

If there is a reason I don't see why this would not work, my apologies and we can close this.