From the peerage.com:
Margaret Buchanan was the daughter of Walter Buchanan.1
She married Alexander Graham, 2nd Earl of Menteith, son of Patrick Graham and Isabel Erskine.1
Her married name became Graham.1
After her marriage, Margaret Buchanan was styled as Countess of Menteith in 1491.

Children of Margaret Buchanan and Alexander Graham, 2nd Earl of Menteith
William Graham, 3rd Earl of Menteith+1 d. c 1543/44
Marion Graham+2

Citations
[S37] BP2003 volume 2, page 2751. See link for full details for this source. Hereinafter cited as. [S37]
[S6286] Clan MacFarlane and associated clans genealogy, online http://www.clanmacfarlanegenealogy.info. Hereinafter cited as Clan MacFarlane.

Chiefs of Clan Buchanan

• 14th – Walter Buchanan,[75][76] (Guthrie Smith identifies him as 8th Chief and Buchanan of Auchmar identifies him as 15th Chief) married Isobel Graham and by her four sons: 1) (Patrick [Guthrie Smith identifies him as 9th Chief but then goes on to advise that the Chiefship passed from Patrick’s father to Patrick’s son[77]and Buchanan of Auchmar identifies him as 16th Chief[76] ] who married the daughter of the Earl of Argyle and by her had George who succeeded his grandfather;2)John first ancestor of cadets of Arnprior;3) Maurice; and 4) Walter first ancestor of cadets of Spittal); and two daughters (Margaret and Elizabeth).
• 15th – George Buchanan,[76][77] (Guthrie Smith identifies him as 10th Chief and Buchanan of Auchmar identifies him as 17th Chief) succeed his grandfather, Walter 14th of Buchanan, in 1526 and died 1560. He first married Margaret Edmonstone and by her a son (John [Guthrie Smith identifies him as 11th Chief but then goes on to advise that the Chiefship passed from John’s father to John’s son[78] and Buchanan of Auchmar identifies him as 18th Chief[79 who married Elizabeth Livingston and by her had George[79] who succeeded his grandfather) and two daughters (Helen and Susanna). Second he married Janet Cunningham and by her had a son (William first ancestor of the cadets of Auchmar) and a daughter (Margaret).
• 16th – Sir George Buchanan,[78][79] (Guthrie Smith identifies him as 12th Chief and Buchanan of Auchmar identifies him as 19th Chief) succeed his grandfather, George 15th of Buchanan, in 1561, married Lady Mary Graham and by her one son (John his successor) and two daughters (Helen and Susanna). http://en.wikipedia.org/wiki/Clan_Buchanan

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Alexander Graham, 2nd Earl of Menteith (c. 1475–c. 1537) married Margaret Buchanan, daughter of Walter Buchanan of the Clan Buchanan, and had two sons and a daughter.
http://en.wikipedia.org/wiki/Alexander_Graham,_2nd_Earl_of_Menteith

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