It's not that hard to understand

A couple of these dudes look like they have no fucking clue what they're doing here

That straight line is how it looks when you only choose to see the evolution of a single species

The “straight line” is not an accurate picture- but neither is the other example. Both are intended to simplify a concept to a point most people can understand it, based on the most relevant and concrete ancestors of a species. In truth, genetic mutations and adaptations abound within a species. The study of evolotuion is a non exact science as there is debate on when we even consider something as a separate species or just a mutation within a species, where the lines blur between extinction and absorption into another species- and so on. The classification of certain extreme examples of a species, phylum, etc shows most clearly where the ambiguities in the system exist. The “line” just shows a concept- that selective gene pairs through repeated breeding create new and novel organisms over time.