Add text for line chart xaxis similar to Bar charts.

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## What did you do?

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## What did you expect to happen?

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## What happened instead?

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## Charts Environment

**Charts version/Branch/Commit Number:**
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**Swift version:**
**Platform(s) running Charts:**
**macOS version running Xcode:**

## Demo Project

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The issue body contains only the submission template, so no file, test, or entry point is identified. Start by locating how Bar charts provide x-axis text and compare that path with line charts. Done means line charts support equivalent x-axis text behavior, with appropriate existing chart-test coverage.

Written by the indexing model from the issue text.