Line graph: Per-data-point (vertical) backgroundColor capabilities

### Feature Proposal

It would be nice to create a method option "backgroundColor" which could return, on a per data-point basis, a backgroundColor so that we could fill a chart like so (example from the exercise game "Zwift"):


This would allow coloring in line graphs to better portray the data within.

I feel like this would have been addressed before in the past. Please let me know if it has and please link me as I haven't been able to find a related issue. I apologize in advance if this has been addressed before.

### Possible Implementation

```
{ ...
datasets: [{
data: [ ... ],
backgroundColor: function(item) {
if (item.y) {
return getColorForY(item.y); // if argument is a datapoint, return a backgroundColor pertinent to the value
}
return 'rgba(0, 0, 0, 0)'; // otherwise return transparent
},
}, ...],
... }
```