StatChart metricLabel migration: conceptual issues with legendFormat mapping

Description

PR #281 introduced by @AntoineThebaud changes to StatChart's Grafana migration to extract metricLabel from legendFormat based on textMode. This issue discusses the conceptual foundation of this approach and its implications.

In Grafana's time-series queries, legendFormat is a template string that controls how series names are rendered in legends and on panels.

• "{{instance}}" - Shows just the instance label
• "Server {{instance}} ({{region}})" - Combines multiple labels in a template
• "Prometheus {{job}}"- Static text with label interpolation

The legendFormat is applied at query time to format the series name based on available labels (if more than one). The series name becomes the visual representation of that time-series.

What is metricLabel in Perses?

In Perses StatChart, metricLabel is a field selector that:

• Selects a specific field/column by name (regex pattern)
• Extracts that field's value from the query result
• Displays that value instead of the calculated numeric result

This maps directly to Grafana's reduceOptions.fields configuration, which specifies which column or field to use for the stat value.

How It Works:

When a query returns data with multiple fields:

Fields: instance, status, version, uptime
Row 1: [server1, up, 1.5.0, 42d]
Row 2: [server2, down, 1.6.0, 15d]

Without metricLabel (or fields in Grafana):

Display the default calculated value (e.g., last value from time-series)

With metricLabel:

For server1 stat: Display "1.5.0" (the version field value)
For server2 stat: Display "1.6.0" (the version field value)

The metricLabel feature exists specifically because Grafana queries can return multiple fields, and sometimes you want to display a non-numeric field value (like version, status, name) as the stat's main value instead of the calculated metric value.

Series Name vs Display Value

legendFormat controls the series name (what appears in legends when showing multiple series)
metricLabel controls the display value (what numeric result is replaced with)
These two features could be used independently or together, but extracting one from the other assumes a 1:1 relationship that doesn't necessarily exist.

Examples

No legendFormat and no metricLabel (thin line on the top is actual series name from very long labels list):

Both legendFormat and metricLabel applied:

The panel.targets Dependency

I found interesting bug where stat chart migration and timeseries migration is relying on panel.targets which is being ommited in grafana.go (perses repo).

The current migration includes:

if #textMode == "name" && (*#panel.targets[0].legendFormat | null) != null {
    metricLabel: strings.Trim(#panel.targets[0].legendFormat, "{}")
}

This depends on panel.targets being present in the migration data. However:

The parent migration script in Perses main repo (grafana.go line ) omits panel.targets when converting panels
This means the condition (*#panel.targets[0].legendFormat | null) will always evaluate to null
The legendFormat extraction never actually occurs in practice. This is especially hard to catch since test input fixture includes targets making it different with actual migration runtime.

Should panel.targets be included in the parent migration?

The same pattern appears in TimeSeries migration with querySettings, creating the same dependency issue. And never migrating querySettings.

Suggestions

Was the legendFormat-to-metricLabel mapping introduced because some dashboards relied on this pattern? What was the actual use case?

I would simplify metricLabel migration relying only on fields from grafana:

#metricLabel: *#panel.options.reduceOptions.fields | null
if #metricLabel != null && #metricLabel != "" {
    metricLabel: #metricLabel
}

And I think we need to keep targets in migration script in order to fix timeseries querySettings migration.

Start by comparing PR #281 with the parent migration in Perses' internal/api/plugin/migrate/grafana.go, especially the panel conversion around line 85. Verify how StatChart and TimeSeries consume targets, legendFormat, and querySettings, then align the migration behavior and fixtures so the relevant fields are preserved and the intended mapping is testable at runtime.