Prevent double initialization of charts

[elalemanyo]

This PR fixes Issue #22, where charts were being initialized multiple times on the same DOM element. This caused unexpected behavior, duplicate event listeners, and unnecessary memory usage.

✅ What’s changed

• Added a guard using getInstanceByDom to check whether a chart instance already exists on the target DOM element.
• Prevents duplicate initialization when the page re-renders (e.g., Turbo/Stimulus reconnect).
• Ensures charts behave consistently across navigation events.

🧪 Why this works

ECharts recommends using lib.getInstanceByDom(chartDom) to detect existing instances. If an instance exists, we skip initialization, avoiding the “double init” problem.