Graft is also known as barratry, and may have an etymological connection to the more familiar word "grift" or grifting. Its exact definition is a bit murky and ambiguous, but it generally deals with illicit sales or bribery, especially in public office. It can also refer to the act of filing too many lawsuits.
The real Dante was accused of graft by his political opponents (it is worth remembering that Florentine politics of the time were notoriously chaotic) and exiled for the rest of his life. This is one grudge that Dante brought into his narrative.
In the text, Dante makes no explicit statement of guilt in Canto 21, where he encounters the pit of barraters, but Virgil does hide him, and the demons seem eager to injure Dante in some way, even if they can't actually throw him into the pitch. This is often taken as an allegory for Dante's real-life struggles with the accusations against him.
I think it's safe to say that Dante is not guilty of graft, but the accusations against him make him feel more vulnerable.