lineChart = vl.markLine({color: "red"})
.data('https://raw.githubusercontent.com/nytimes/covid-19-data/master/us.csv')
.transform(
vl.filter('month(datum.date) >=2'),
vl.window(
vl.first_value('cases').as('previous_cases')
).frame(-1, 0),
vl.calculate('datum.cases - datum.previous_cases').as('new_cases'),
vl.window(vl.mean('new_cases').as('rolling_mean')).frame(-6,0)
)
.encode(
vl.y().field('rolling_mean').type('quantitative').title('Cases'),
vl.x().field('date').type('temporal').title('Date')
)
.title('Daily 7-Day Average of new U.S. Covie cases')
.height(400)
.width(width)