I looked at your Excel and for starters the periods are period intervals and not actual dates. Tableau sits in that 'give me raw data' state. In order to get date points for each year to be that average level (to make it easier to do this at least) you would need a data point for each year. The issue is that 1660-1700 can not be interpreted as a date. Now 1660 can and so can 1700 but together at best they are a string to tableau (line of text).
Try using this table:
Year Average Price of Goods Aggregated Price for Non-Assesed Return Cargos 1660 18635 689495 1701 28193 1296878 1751 53112 318672 1761 29187 379431 1775 68396 6018848 1791 37840 794640 1796 51097 1532910 1800 12451 1220198
Because a new value is at 1701 you do not need to assign that range inside your data columns.In order to get a graph with a flat line from 1660-1700 it would take some data work or clever table calculations within Tableau.
DatePeriods.twbx 17.8 KB