Grafana stacked panels should render null values as zero
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component/observability
contribution
first-time-contributor
type/enhancement
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- Go
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Description
## Problem
Some TiDB Grafana dashboards use stacked graph panels. When a stacked panel keeps the default null handling (`nullPointMode: "null"` or connected gaps), missing points in one series can make the stacked visualization misleading.
For stacked panels, null values should be rendered as zero so gaps in one series do not corrupt the visual stacking of the remaining series.
## Expected contract
For every legacy Grafana graph panel with stack enabled:
- panel or series override has `"stack": true`
- the same panel should use `"nullPointMode": "null as zero"`
Non-stacked panels should not be changed only because they have `nullPointMode`.
## Related fix
PR: https://github.com/pingcap/tidb/pull/68219
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