X Axis tick aggregation

Daily tracking is great but sometime it feels better to track habits on a weekly basis, while keeping a daily note routine.

What would you think about adding a "xAxisAggregate" attribute that works in pair with "xAxisTickInterval"?

For instance in a bar chart:
```
xAxisTickInterval: 1w
xAxisAggregate: sum
```

would sum all the elements for each X tick use that as the bar value.



