fix(viz): display offset metrics as dashed lines regardless of compar…

[anumukul]

SUMMARY

Fixes #36806

This PR fixes a regression where offset metrics (time comparisons like "1 year ago") were not displayed as dashed lines when charts have no dimensions.

Root Cause: The isDerivedSeries function only applied dashed styling when comparison_type === "Values", and hasTimeOffset didn't detect series where the name exactly matched a time offset (which happens when there are no dimensions).

Changes:

• Removed the restrictive comparison_type === "Values" check in isDerivedSeries.ts
• Added exact match handling in timeOffset.ts for dimension-less charts
• Updated tests to reflect new behavior

BEFORE/AFTER

BEFORE

**AFTER **

TESTING INSTRUCTIONS

1. Create a new Line Chart
2. Select a dataset with a time column (e.g., birth_names with ds)
3. Add a metric (e.g., COUNT(*))
4. Set Time Range to a valid range (e.g., 2005-01-01 to 2008-12-31)
5. In Advanced Analytics, add Time Comparison: 1 year ago
6. Do NOT add any dimensions (leave Dimensions empty)
7. Click "Update Chart"
8. Verify the offset metric line (1 year ago) appears dashed, not solid

ADDITIONAL INFORMATION

• [x] Has associated issue: Fixes #36806
• [ ] Required feature flags: None
• [x] Changes UI
• [ ] Includes DB Migration
  • [ ] Migration is atomic, supports rollback & is backwards-compatible
  • [ ] Confirm DB migration upgrade and downgrade tested
  • [ ] Runtime estimates and downtime expectations provided
• [ ] Introduces new feature or API
• [ ] Removes existing feature or API