Is there a visualisation tool for discarded block chains in Bitcoin?
There are several sites that maintain this information, and Bitcoin Core can be asked about it using the getchaintips RPC command.

It is however important to know that such information can never be guaranteed to be complete. Nodes only relay new blocks that are part of the (new) best chain.

A fork appears when two blocks are produced at approximately the same time. They both propagate across the network, and one part of the network sees block A first, and another sees B first. Assuming A eventually wins (is extended first), nodes that saw A first and were not connected to a peer that accepted B first, will never see B.