LineChartView crashes on a single value

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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:**
**Xcode version:**
**Swift version:**
**Platform(s) running Charts:**
**macOS version running Xcode:**

## Demo Project

ℹ Please link to or upload a project we can download that reproduces the issue.

Start at the LineChartView entry point and reproduce the crash with a single value. The issue provides no environment, stack trace, demo project, file, or test; done would require a confirmed fix for that reproduction and evidence that the single-value case no longer crashes.

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