I'm trying to make various graphs based on the function "y = tan(x)" for
revision purposes. The only problem is that the graph contains an asymptote:
a dotted line parallel to the y-axis that the plot-line approaches but never
quite reaches. Excel doesn't seem to recognise this and I don't know how to
make it deal with asymptotes.

In other words, the asymptote represents a value of +-infinity.