You can choose to limit the values of the dimension that are shown by changing the Limitation setting. In your case you want Fixed Number and Top. As value you can use a variable. To change the variable you can use an extension like qsVariable which has a Slider layout option. You can see an example below which uses a variable vTopX that is set with the qsVariable slider:
gwassenaar That's excellent by the way and the qsvariable is very useful. However the issue gets slightly more complicated as the customers are in a table prioritised based on spend (the group I want to limit to say 10 or 20) and the Tree Map is based on diversity of products purchased by that customer which I want limited by the top 10 or 20 spenders. This way I can see which of the top customers I need to target, as if they are ranked low, they may be spending a lot but only on 1 specific product, and therefore they are a great target for cross sell.
If I apply the limitation to the table only, then all customers are still in the Tree Map, if I apply to the Tree Map as well, it gives me my top 20 customers based on product mix rank, not product spend rank.
If I hand select in the table, it reflects correctly in the Tree Map.