Bug report: Legend charts displays event name instead of series custom name

[slshults]

Bug Description

Bug description

The chart legend is displaying the event name instead of the custom name given to the series

How to reproduce

1. Edit an existing insight, or create a new insight
2. Select a chart for the time series
3. Under Detailed results click the edit icon and set custom names for one or more series
4. Click the Options menu for the chart and tick the box for Show legend

Additional context

From: https://posthoghelp.zendesk.com/agent/tickets/12985

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]