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May 21, 2024
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adopt and document the new apache-arrow date encoding #1375

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adopt and document the new apache-arrow date encoding
Fil May 21, 2024
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mbostock May 21, 2024
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7 changes: 7 additions & 0 deletions docs/lib/arrow.md
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Expand Up @@ -60,13 +60,20 @@ Visualized with [Plot’s difference mark](https://observablehq.com/plot/marks/d

```js echo
Plot.plot({
x: {type: "utc"},
marks: [
Plot.ruleY([0]),
Plot.differenceY(table, {x: "date", y: "value"})
]
})
```

<div class="note">

The chart above specifies _x_ as a UTC scale because Apache Arrow represents dates as numbers (milliseconds since [Unix epoch](<https://en.wikipedia.org/wiki/Epoch_(computing)>)) rather than Date objects; without this hint, Plot would assume that _date_ column is quantitative rather than temporal and produce a less legible axis.

</div>

## Apache Parquet

The [Apache Parquet](https://parquet.apache.org/) format is optimized for storage and transfer. To load a Parquet file — such as this sample of 250,000 stars from the [Gaia Star Catalog](https://observablehq.com/@cmudig/peeking-into-the-gaia-star-catalog) — use [`FileAttachment`](../files). This is implemented using Kyle Barron’s [parquet-wasm](https://kylebarron.dev/parquet-wasm/) library.
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