Obtaining identifier from plot of sequence GC%
The tricky bit here is that you need a list of sequence records sorted in the same order as the CG values. You can accomplish this by making tuples of the seq records and GC values, then sorting the tuples specifying the GC values as the sorting key:

gc_w_records = sorted(((GC(rec.seq), rec) for rec in SeqIO.parse("ls_orchid.fasta", "fasta")), key=lambda tup: tup[0])
gc_values = [x[0] for x in gc_w_records]
records = [x[1] for x in gc_w_records] 


I don't know much about matplotlib and it's going to be challenging to display 100 IDs, but you'll probably be interested in the extremes of the GC distribution, which you can obtain directly from the sorted list of records:

records[0:2]
# [SeqRecord(seq=Seq('CGTCACGAGGTCTCCGGATGTGACCCTGCGGAAGGATCATTGTTGAGATCACAT...CAT', SingleLetterAlphabet()), id='gi|2765587|emb|Z78462.1|PSZ78462', name='gi|2765587|emb|Z78462.1|PSZ78462', description='gi|2765587|emb|Z78462.1|PSZ78462 P.sukhakulii 5.8S rRNA gene and ITS1 and ITS2 DNA', dbxrefs=[]),
# SeqRecord(seq=Seq('CGTAACAAGGTTTCCGTAGGTGAACCTGCGGAAGGATCATTGTTGAGATCACAT...AGG', SingleLetterAlphabet()), id='gi|2765568|emb|Z78443.1|PLZ78443', name='gi|2765568|emb|Z78443.1|PLZ78443', description='gi|2765568|emb|Z78443.1|PLZ78443 P.lawrenceanum 5.8S rRNA gene and ITS1 and ITS2 DNA', dbxrefs=[])]


or:

records[-3:-1]
# [SeqRecord(seq=Seq('CGTAACAAGGTTTCCGTAGGTGAACCTGCGGAAGGATCATTGTTGAGACAGCAG...TAA', SingleLetterAlphabet()), id='gi|2765656|emb|Z78531.1|CFZ78531', name='gi|2765656|emb|Z78531.1|CFZ78531', description='gi|2765656|emb|Z78531.1|CFZ78531 C.fasciculatum 5.8S rRNA gene and ITS1 and ITS2 DNA', dbxrefs=[]),
# SeqRecord(seq=Seq('CGTAACAAGGTTTCCGTAGGTGAACCTGCGGAAGGCTCATTGTTGAGACCGCAA...AAG', SingleLetterAlphabet()), id='gi|2765625|emb|Z78500.1|PWZ78500', name='gi|2765625|emb|Z78500.1|PWZ78500', description='gi|2765625|emb|Z78500.1|PWZ78500 P.warszewiczianum 5.8S rRNA gene and ITS1 and ITS2 DNA', dbxrefs=[])]