That is going to prove really difficult to do, so let me offer a native solution.
Make a summary report to rank customers by sales and have a running % down a column columns. To do that you put the field on the summary report twice, once regular and once set to display as a running total.
Take note of what Activity value gets you to the the 1% point. Say all customers over 5000 in Activity.
Make a table with a single record. It will be record ID# of 1.
Related that to the Customer list via a formula field with a formula of 1.
Make a field where you manually enter that 5000. Call that field Activity 1% cutoff.
Lookup that [Activity 1% cutoff] down to each customer. Make a checkbox formal field to calculate if the customer activity is > 5000.
Chart only sales or customers or activity where that checkbox is true.
My point is that it will be difficulty to automate so so that cut off manually from time to time.