Nobody ever settled which acts are the grave ones, and the reason is that two methods were available and both had warrant.
The first works from an enumeration. A report received in both of the two most widely transmitted hadith collections has the Prophet warn against seven destroying things, and names them: associating anything with God, sorcery, killing a person whom God has made inviolable except by right, consuming interest, consuming the property of an orphan, turning one's back on the day of battle, and slandering chaste believing women who are unaware of what is charged against them. Scholars who took that list as the definition had a short and clear category.
The second works from a criterion. On this approach the seven are illustrative, and a grave sin is any act to which the Quran or the hadith attaches a fixed legal penalty, or a specific threat of hellfire, of curse, or of divine anger. That test can be applied to the whole corpus, and it produces a very much longer list. A manual circulated under the name of al-Dhahabi, who died in the fourteenth century, enumerates far beyond seven, and Ibn Hajar al-Haytami's sixteenth century al-Zawajir, whose title announces it as a work of deterrence from committing the grave sins, runs into the hundreds.
The atlas carries the disagreement rather than picking a side, because both positions are classical and both are still argued. It is worth separating this question from the older and sharper one over what an unrepented grave sin does to a person's standing as a believer, which divided the Khawarij, the Mu'tazila and the Murji'a in the first Islamic centuries and is treated under Tawbah.