Zam, this should be quite easy. You just need to create a new calculated field that calculates the average for ALL sales rep.
Let me walk you through it:
I used your data table and then added it to my PowerPivot (Excel 2013). Then I created those calculated measures:
1. Sales Average:
=AVERAGE(SalesData[SalesGP])
2. Sales Average ALL -- this will calculate the average for ALL rows in the table and will be used in other calculations. Notice I used the first calculated field as the first parameter to make this flexible:
=CALCULATE([Sales Amount Average],ALL(SalesData))
3. Sales Comparison to Average. I wasn't sure what is your goal, but I made this one a bit more complex as I wanted to display the performance in percentage:
=IF([Sales Amount Average] > 0, [Sales Amount Average] / [Sales Average ALL] -1)
Basically, what this does is that first it checks if their an existing calculation for a sales representative, if so then it divides Sales Average for a given sales representative by the average sale amount for ALL sales representatives. And then I subtract 1 so the performance can be easily grasped just by looking at the percentages.
To make it easy-to-understand, I decided to use bar charts in conditional formatting instead of stand-alone pivotchart -- I believe it does exactly what you need:
In the picture above, the first table represents your data. The second table is the actual powerpivot table and shows what I have described.
Hope this helps!
EDIT
I didn't want to make things over-complicated, but should you want to remove the percentage total from Grand Totals row, use this calculation instead for the percentage comparison:
=IF(HASONEVALUE(SalesData[SalesRep]),
IF([Sales Amount Average] > 0,
[Sales Amount Average] / [Sales Average ALL] -1),
BLANK()
)
EDIT - ADDED AVERAGE COMPARISON FOR WORKGROUPS
To calculate the performance per workgroup instead of ALL sales representative, add those two measures:
4. Sales Average per Workgroup
=CALCULATE(AVERAGE(SalesData[SalesGP]),ALL(SalesData[SalesRep]))
This will calculate the average sale per workgroup (don't get confused with using SalesRep in ALL function, it's related to filter context).
5. Sales Diff to Average per Workgroup
=IF(HASONEVALUE(SalesData[SalesRep]),IF([Sales Amount Average] > 0, [Sales Amount Average] - [Sales Average per Workgroup]),BLANK())
This simply calculates the difference between average sale of given sales rep and the average sale per workgroup. The result could then look like this:
I have uploaded the source file to my public Dropbox folder.