Workout Wednesday: Sales for the Last N Periods vs. Prior Year

I'm going back to Workout Wednesday week 16. Why? Because I really struggled with it. I was so close for so long, but couldn't quite get my date calculations correct. After writing them down on paper and building tables in Tableau to verify I had them correct, the rest was pretty straightforward.

This use case is super useful in a business context. I like Workout Wednesday challenges that you can employ later. The most important requirements:

1. Use a date parameter to select a select end date, limit it to all days in 2017.
2. Use a parameter to select the period type (day, week, or month).
3. Use a parameter to select the number of periods to go back (limit from 1 to 12).
4. Create a bar chart that show the total sales for the last complete period. 
5. Add sales for the same period as a label on the end of the bars.
6. Compare the selected period to sales over the same period from the prior year.
7. Add a blue arrow up if sales are up compared to prior year.
8. Add a red arrow down if sales are down compared to prior year.
9. Show the difference in sales over these time periods. Make sure to show no negative signs. The arrows will indicate the change.