I use a lot of charts that compare last X days with the last X days before. So I get 2 series in 1 chart. But the tooltip always shows the date from the X-axis.
See example. The dates of last 28 days are used on the X-axis, the light blue line is the comparison, and contains older data (28 days prior to the last 28 days). But in the tooltip I keep seeing the value of the X-axis, in this example 'Jun 23'. It would be great to have the actual date here that matches the serie you hover. That way it also works in Analytics.