Maybe you can try this :
In the chart properties "Sort" tab, select your expression and select "Expression" in sort by. Here you can sort Descending (from highest to lowest sales) or Ascending (from lowest to highest sales) and then enter the expression.
In the "Dimension Limits" tab, choose "Restrict which values are displayed ..." and select to show only the largest or smallest 20.
So, in fact, you need a table to show the highest 20 and an other table to show the smallest 20.
If you used two objects (for example straight tables) you could set those top/bottom 20 within the limitation tab. Within one object it's more complicated and you would need a calculated dimension like this:
if(aggr(rank(sum(Value), 4), Customer) < 21 or
aggr(rank(sum(Value), 4), Customer) > $(=count(distinct Customer)) - 20, Customer),
aggr(rank(sum(Value), 4), Customer))
and as expression simply: sum(Value).