Constellation diagram of FSK
FSK is difficult to visualize as you increase in order. The reason for this is when you are using FSK, you have orthogonal frequencies that essentially add an extra dimension to your plane. You can visualize up to 3d (3 frequencies) as shown below (pardon the hand paint drawing), but once you get greater then that FSK just can't be represented this way. However, just because we aren't able to plot it on a graph, the matrix math still applies equally the same, just with added dimensions.