I think this is too much modesty, or a kind of self-undercutting to try to convey the importance of the point. But functionally it undercuts the significance of the earlier parts of the chapter. At the end here I'm understanding the argument as being
- The antislavery campaign shows what state-of-the-art data visualization meant c. 1800, and these two different visualizations from Clarkson make the case that he should be considered one of the canonical figures.
- That's important because it heightens a set of ethical and political questions about whether and when to visualize. Clarkson's work can be considered a countervisualization or something -- possibly a concept to introduce ? -- because it's taking advantage of the trade etc. Also highlights dataviz as a political-rhetorical form, not just a scientific practice about astronomy etc.
- Just because we admire things about Clarkson's career doesn't mean should literally canonize him as a saint. Equiano's reaction shows that even at that time there are a different set of requirements.
And then there is the metaphor of water and streams. This does a few things:
1. provides a counterpoint to the God's-eye, object view by adding a contingency of flow and direction, fluidity, and contingency.
2. was useful for ~1800 readers who ALSO weren't always looking for this objective god's-eye view, which is OK. (I think the infographic/dataviz distinction from the introduction here is useful, because it underlines that the more 'subjective' or whatever flow timelines are an ADVANCE on Priestley's straight lines and can be seen as such.
3. Motivates your own data visualization of the streams of with the also-canonical Mississippi visualization. I may have missed this but I think the connection here is almost fully implicit. This could be one key to motivating the water thing as your own choice.