For relational data sources, dates and times are automatically placed in the Dimensions area of the Data pane and are identified by the date or date-time icon. For example, the Order Date and Ship Date dimensions from an Excel data source are shown below:

When you place a relational date on a shelf, the field name is automatically modified to reflect the default date level. Tableau defines the default date level to be the level at which there are multiple instances. For example, if the date field includes multiple years, the default level is year. However, if the date field contains data for just one year but includes multiple months, then the default level is month.

Tableau Public is a free service that lets anyone publish interactive data visualizations to the web. Visualizations that have been published to Tableau Public ("vizzes") can be embedded into web pages and blogs, they can be shared via social media or email, and they can be made available for download to other users.
Check it out : https://public.tableau.com/en-us/s/

You can add a reference line to any continuous axis in the view.
To add a reference line:
Drag Reference Line from the Analytics pane into the view. Tableau shows the possible destinations. The range of choices varies depending on the type of item and the current view.
In a simple case, the drop target area offers three options:

This is true. a Manual Sort lets you select a value and move it to the desired position, either by dragging it in the list or using the arrows to the right.
However, as soon as you choose some other type of sort - be it field, nested, or alphabetic, our custom created manual sort gets deleted/cleared.

Unioning is the correct answer!
From the official documentation:

For example, suppose you have the following customer purchase information stored in three tables, separated by month. The table names are "May2016" , "June2016" and "July2016."