If you are able to share a workbook that would be great! It is always easier to help out when we can see the structure of the data.
It sounds like in your final line graph, the x-axis will be [LAST SEEN ON], and for all dates in a week, the [End Date] should be the Friday of that week. If that is the case, then you might create a calculation for [End Date] like:
DATEADD('day', 5, DATETRUNC('week', [LAST SEEN ON]))
DATETRUNC() will return the first day of the week that any date belongs to, which is Sunday by default. Then DATEADD() will add five days to move the end date to Friday. See Date Functions for more information.
Hope this helps!