"Can you interface excel to the serial port"

Yes, I've seen products advertised to do that.
Try keywords like "Excel", "RS232", and "wedge" in your favourite internet search engine.

Also look on instrumentation sites; eg, Amplicon, National Instruments.

Borland C++ Builder (BCB) has some MS-Office interface stuff, though I've not tried it myself.

BCB also has some graphing components of its own.

You can also get devices that plug between the keyboard & PC, so that the serial data appears as if it were typed at the keyboard. (I think that'd mean your serial data would have to be very carefully formatted for your Excel spreadsheet?)

The simplest, no-programming, approach is just to capture the data to a file with a terminal emulator (Hypoterminal if you really must), then import that file into Excel.