I'm assuming your data is arranged as follows:
Column A has a header called "Name" and the name of each salesman in it.
Columns B to G have a month header e.g. Jan, Feb etc.
The rest of the data is a sale amount for each salesman.
Select the data and from the menu bar select Data then Pivot Table and Pivot Chart Report.
Follow the wizard through making sure you select Pivot Chart Report (with Pivot Table Report) in the first step.
From the chart type select Pie Chart.
Drag the "Name" item to Drop Page Fields Here and drag each month to Drop Data Items Here.
If you select a name from the drop down list, the chart will update automatically.