In Ancient Greece they didn't have a proper scale for graphing things, later introduced by René Descartes and is now known as coordinate graphing, so to visualize parabolas and more complicated 2D shapes they would imagine slices in 3D objects, typically a cone. This is one of the reasons the Greeks are known for their mathematical achievements, as they were one of the first societies to both value mathematicians and to achieve notable abstraction in their math.
This image represents how Greek Mathematicians viewed more complicated 2D shapes, using the cutting of 3D shapes to visualize the concepts so they could figure out equations to represent area and further analyze the shapes. I credited the creation of the modern day graphing system, most typically known as analytical geometry, with René Descartes, but it could also be attributed to Pierre de Fermat, as they founded the concept at the same time. The founding of analytical geometry itself is attributed typically almost 2000 years before in 200 BC by Apollonius of Perga for his work with conics, defined by a plane going through a cone, introducing the very concept I refer to in this post. If you've read this entire description, I thank you. Expect a meme of some form relating to these concepts in the near future, however I will not be the one making it.