28-day average smoothing should average with earlier data than time period

Feature request

Is your feature request related to a problem?

The smoothing can be a bit misleading early on because it's averaging less data.

It looks like the way this works currently is that the first ~27 days are averaged over fewer days. For example, with daily data and 28-day smoothing:

• Day 1: average of just day 1 (= raw value)
• Day 2: average of days 1–2
• Day 3: average of days 1–3
• ...
• Day 28+: full 28-day trailing average

Describe the solution you'd like

Pull more data ahead of the chosen time period but only use the data from the time period.

Describe alternatives you've considered

Additional context

Ticket: https://posthoghelp.zendesk.com/agent/tickets/53621 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/55493)

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

The issue names no files, entry points, or tests. Start by locating the analytics smoothing implementation and its existing test coverage; verify how the selected period and 28-day window are calculated. Done means early points use earlier data for smoothing while only values within the chosen period are displayed, with tests covering the initial days.