Update README.md

Author: mbostock

README.md

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 [**Observable Plot**](https://observablehq.com/plot/) is a free, [open-source](./LICENSE), JavaScript library for visualizing tabular data, focused on accelerating exploratory data analysis. It has a concise, memorable, yet expressive API, featuring [scales](https://observablehq.com/plot/features/scales) and [layered marks](https://observablehq.com/plot/features/marks) in the *grammar of graphics* style.
 
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+  <source media="(prefers-color-scheme: dark)" srcset="https://observablehq.observablehq.cloud/oss-analytics/@observablehq/plot/downloads-dark.svg">
+  <img alt="Daily downloads of Observable Plot" src="https://observablehq.observablehq.cloud/oss-analytics/@observablehq/plot/downloads.svg">
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+<sub>Daily downloads of Observable Plot · [oss-analytics](https://github.com/observablehq/oss-analytics/)</sub>
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 ## Documentation 📚
 
 https://observablehq.com/plot/