compare chart should be a grouped bar chart

[Kilian]

Describe the feature or enhancement

The compare chart shows a bar chart overlaid with a line chart, with the bars being for this period and the lines being for the previous period.

Comparing bars with lines is difficult to do, and also doesn't adhere to guidelines like those by Stephen Few and Edward Tufte.

Instead, the compare chart should be a stacked grouped bar chart, where for each day/tick there are two stacked bars visible: the one from the previous period (having visitors and view stacked) and the one from the current period. This makes it significantly easier to compare the two as shown in this chart.js example: https://www.chartjs.org/docs/latest/samples/bar/stacked-groups.html