Improve display of Net Revenue and Amount Refunded

**Is your feature request related to a problem? Please describe.**
When looking at the Sales report graph under _LifterLMS > Reporting_, "Sales" tab, points are joined together. This makes it look like on days with no revenue or refunds, that there was actually a refund or revenue.

**Describe the solution you'd like**
Turn that part of the graph into another format, for example points or bars. This would be this line in llms-analytics.js

```
type: ( 'count' === type ) ? 'bars' : 'line',
```

and changing it to `bars` always even if `type` is `amount` or else some other format.

**Describe alternatives you've considered**
Adding points to the line so at least you can see where the actual data is. Or perhaps filling in the dates, so we put `0` explicitly for dates with no data.