Mike Freeman made a helper function that adds that sort of animation (data gradually coming in) using D3 atop Plot: https://observablehq.com/@mkfreeman/plot-animation
You could upvote this incremental rendering issue: https://github.com/observablehq/plot/issues/328
Or this one for adding a “time channel” to do data-driven animation (for which there’s an old draft PR): https://github.com/observablehq/plot/issues/166