For stock prices, starting the y axis wherever is aesthetically pleasing makes some sense because everybody will have a different non-zero cost basis for their investment, and the graphs need to be able to clearly depict fluctuations that are minor on a percentage basis. For something like the em-dash prevalence on HN, the most meaningful question is whether it has doubled, tripled, or whatever relative to the pre-LLM corpus, and that's most clearly visually depicted by starting the y axis at precisely zero.