[Simple Analytics] Subscriber Change Over Time Chart & Mailchimp API Integration

[iamdharmesh]

Description of the Change

The PR adds a Subscriber change over time section on the Analytics admin page, a diverging bar chart (new subscribers vs unsubscribes) and a totals doughnut with net change, driven by the Mailchimp Marketing API lists/{list_id}/activity endpoint. The section loads on the existing mailchimp-analytics-refresh event so it stays independent from KPIs and Form Performance section failures in this request do not block other analytics UI.

Technical Details:

Backend:

• New Mailchimp_Subscriber_Activity class (includes/class-mailchimp-subscriber-activity.php): admin AJAX action mailchimp_sf_get_subscriber_activity, capability check (MCSF_CAP_THRESHOLD), nonce (mailchimp_sf_analytics_admin_nonce), and validated list_id / date_from / date_to (Y-m-d).
• Fetches raw activity (180-day API window), normalizes to daily series, then filters to the requested range intersected with the last 180 days (site timezone). Returns limited: true when the selected span exceeds what Mailchimp can supply so the UI can show an info notice.
• Bucket interval follows the plan: daily (≤30d), weekly ISO Mon–Sun (31–90d), monthly (91d–1y).
• Transient cache (15 minutes) on successful raw responses only; key scoped by list (mailchimp_sf_subscriber_activity_ + hash) so one cache entry serves many date-range requests.
• Registered and init()’d from mailchimp.php next to existing analytics classes.

Frontend:

• includes/admin/templates/analytics.php: new card markup for the section (title, date subtitle, notice, chart + totals, loading/error/empty states). Strings use the mailchimp text domain.
• assets/js/analytics.js: subscribes to filter refresh, POSTs to admin-ajax.php, renders Chart.js bar (diverging: positive new subs, negative unsubs) and doughnut for range totals; handles empty, error, and limited-range notice.
• assets/css/analytics.scss (and built dist/ assets via npm run build): layout (chart + totals column), legend, notice/error/loading states, responsive behavior.

Closes #200

How to test the Change

1. Go to Mailchimp > Analytics.
2. Ensure the "Subscriber Change Over Time" graph is displayed, including both bar and doughnut charts.
3. Verify that changing the list and date presets (e.g., 7 / 30 / 90 / 180 / 365 / custom) refreshes the graph, and that buckets and labels match the aggregation rules.
4. Select a longer date range (>180 days) and confirm that a limitation notice is shown and only ~180 days of data are displayed in the bars.
5. Ensure the section shows the correct loading state.
6. Confirm that an appropriate error message is displayed in case of API failure (e.g., disconnected account or invalid credentials).
7. Check responsiveness and cross-browser compatibility.

Changelog Entry

Added - Subscriber Change Over Time chart to the Analytics page.

Credits

Props @iamdharmesh @builtbytay April Domingo Cameron Campbell

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[Copilot]

Pull request overview

Adds a new “Subscriber change over time” analytics section (bar + doughnut charts) backed by a new admin AJAX endpoint that fetches Mailchimp list activity and aggregates it for charting.

Changes:

• Register a new backend data provider (Mailchimp_Subscriber_Activity) and hook an admin AJAX action for subscriber activity.
• Add new Analytics admin template markup for the subscriber activity card (states, charts, totals).
• Extend the analytics JS/CSS to fetch and render the diverging bar chart + totals doughnut, and update styling via SCSS.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
mailchimp.php	Boots the new subscriber activity provider class.
includes/class-mailchimp-subscriber-activity.php	New AJAX endpoint + caching + date-range filtering/aggregation for Mailchimp activity data.
includes/admin/templates/analytics.php	Adds subscriber activity section markup; adjusts deep link button styling.
assets/js/analytics.js	Adds subscriber activity module (Chart.js rendering) and introduces WP i18n usage.
assets/css/analytics.scss	New SCSS-based styling for analytics page and subscriber activity card.
assets/css/analytics.css	Removes the previously committed compiled CSS source file.

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[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

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[alaca]

Looks good 👍