offset y axis by value

### Feature Proposal

My suggestion is that the offset becomes an number. Right now I try to change ```axis: Scale, axis._startValue,
axis._endValue, axis._valueRange``` but I am overwritten evertime something is drawed. Maybe there is way but it is very difficult. I need to offset the y-axis by a lot more than the standart true gives me.
```
y: {
offset: boolean
}
becomes
y: {
offset: number
}
```

### Possible Implementation

Change offset to take a value.