chart_v4 = GroupedBarChart(data_time, {
x: d => d.destination,
y: d => d.time == 0 ? 0 : d.distance/d.time,
z: d => d.grp,
xDomain: d3.groupSort(data_time, d => d.emission, d => d.destination),
yLabel: "↑ Speed (in km/h)",
zDomain: grp_time,
colors: d3.schemeCategory10.slice(0,grp_time.length),
width,
height: 300
})