Retention queries are not saved correctly to dashboard

[macobo]

Bug description

Client reported that after saving a retention graph to dashboard, the graph is reset to defaults ($pageview -> $pageview)

Expected behavior

The correct retention graph is shown on dashboard.

How to reproduce

1. Go to insights -> retention
2. Add retention from two custom events
3. Save to dashboard
4. Visit dashboard
5. See the graph is wrong

Environment

• PostHog Cloud
• self-hosted PostHog, version/commit: please provide

Additional context

Thank you for your bug report – we love squashing them!

[u-ashish]

+1 to this, also noticed on my dashboards

[timgl]

Fixed in #2884