First of all I would recommend that you split the Time Period field in the script. So you have a Min Time Period and a Max Time Period by Visitor. So your script would look something like this.
Date(Min([Time Period])) AS MinDate,
Date(Max([Time Period])) AS MaxDate,
(ooxml, embedded labels, header is 1 lines, table is [IS1])
Group By Visitor
To achieve what you are looking for.
Right click on the tab > New Sheet Object > Chart > Select Bar Chart > Select Next >Dimensions > Select Add Calculated Dimension > Add the following expression > MaxDate - MinDate > Select Ok > Select Next > Add the following Expression in the Expression window > Count(Visitor) > Select Ok
Hope this helps.