Hey callum dunne,
The general way to do this is to set up a table calculation that will flag the desired marks, and put that calculation on the Filter Shelf. Filters based on table calculations are performed after most all other calculations are completed, so they effectively hide results without changing them.
One caveat is with regards to Tableau's built-in reference line calcs. Those built-in computations are performed after table calc filters are applied (see this post for details: http://community.tableau.com/thread/113542#comment-39603), so that would exclude the "hidden" values from being used in the reference lines. The workaround is to build your own calculations that duplicate what you want Tableau to do, put those calcs on the Level of Detail shelf, and use those for the reference lines instead.
Hope this help!
Lénaïc RIÉDINGER, Global Community Engineer Tableau
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