Giving values to filtered column

I would do this by adding a calculated column that categorizes TotalAmount into levels using the SWITCH function.  See the example at http://www.powerpivotpro.com/2012/06/dax-making-the-case-for-switch/ for info on SWITCH.  Your formula would be something like:

Level=SWITCH(TRUE(),
             [TotalAmount]<7500, “Lvl1”,

             AND([TotalAmount]>=7500, [TotalAmount]<20000), “Lvl2”,

             AND([TotalAmount]>=20000, [TotalAmount]<=50000), “Lvl3”,
             “Lvl4”
           )

Then you would add a measure that counts that levels and that is what you would use on your chart.  It would look something like:

LevelCount = CALCULATE(COUNTA([Level]))