Add dates for previous period in tooltip

[annaszell]

Feature request

Is your feature request related to a problem?

When previous period comparison is selected in Insights, users have to do the mental math to figure out which dates the data points refer to.

Describe the solution you'd like

When hovering over a data point, the tooltip should include the date for the previous period, not just the value.

Describe alternatives you've considered

Doing the mental math 😅