Demo of [Perspective](https://github.com/perspective-dev/perspective), using US
COVID data from [https://covidtracking.com](https://covidtracking.com/).

The `Group By` column is a good example of using Perspective's ExprTK column
language to clean/reformat a column. The raw "date" column, as imported from
[https://covidtracking.com](https://covidtracking.com/), use the date format
`YYYYMMDD`. Using this expression, we can parse this format into a well-typed
`date()` column, then bucket this by week to get a sensible graph:

```
// Parsed \"date\" bucket by week
var year := integer(floor(\"date\" / 10000));
var month := integer(floor(\"date\" / 100)) - year * 100;
var day := integer(\"date\" % 100);
bucket(date(year, month, day), 'W')
```
