A year-to-date comparison needs the same cutoff in both years. If June is the latest complete month, compare January through June this year with January through June last year. A full prior year would make the current year look weak just because it's shorter.
Use a parameter to make the reporting cutoff explicit. The examples use an integer month for monthly reporting. For a daily report, use a date parameter and shift it back with DATEADD, then include both the start and end of each interval. A month-end report and a report through the middle of a month answer different questions.
Calculate growth from the two totals. A zero prior-period total has no defined percentage growth, so return Null instead of displaying a misleading zero. LOOKUP is useful for adjacent visible periods, but it follows the view's order and can't repair unequal date windows.