While the series does often go its own way and deviate from the comics, it still makes sure to hit the major beats of the graphic novels, either in exact copycat or slightly altered fashion.
This started with the death of Glenn, which comes from the major introduction of Negan in the 100th issue (albeit for slightly different reasons, as Abraham is the first one killed in the show).
Then you've got a number of other key moments, such as Carl showing Negan his eye and the villain's surprising response, the Spencer 'guts' death, and many lines of Negan's dialogue. This continued in the second-half, with Sasha taking on the role of Holly from the comics in infiltrating the Sanctuary, and turning into a zombie (although again this played out differently, as her being a walker surprised Negan, whereas in the comics he presented her to Alexandria with a bag over her head).
There was also Dwight's turn to the 'good' side, and all of the communities coming together, with Maggie in a leadership role, to take on the Saviours.