Thanks for the observation. As you deduced, that is a feature that is not yet available in the online client of the post-processor. We will be looking into expanding filter options in the following period, but this functionality is not present as of right now.
If I understand correctly what you are trying to do, I may, for the time being, suggest using separate slice instances, for the different coordinates you require, then taking a screenshot with the built-in functionality.
Let us know if we can further assist you.