There are several differences between NaNs and DomainExclusions in functional series:
1) Currently DomainExclusions work, while NaNs do not
2) Automatically extracting domain from arbitrary function is unreliable -- this is quite similar to the reason I did not want to implement automatic extent. Finding discontinuity will be predicated on trying to calculate value inside of it, which may be unlikely, especially for single-point exclusions.
3) Once found, it is not entirely obvious what to do -- actual point of discontinuity is somewhere between current point and previously drawn one. Ideally, chart should use binary search to find it.
4) Raising exceptions is slow. So finding domain based on exceptions may be inefficient for functions like Log and Sqrt.
Nevertheless, all of the above is "just a matter of programming", so if it is required,
I'll consider implementing it.
Additionally, I have thought about adding discontinuity marks, but postponed it
until somebody needs them.