Understanding the Ghost Chart Rendering Bug

Background

Chart.js relies on an imperative rendering model using HTML5 canvas, which must be manually cleaned up. In modern SPA frameworks where virtual DOM diffs drive UI updates, Chart.js instances can persist invisibly in memory or re-render unexpectedly if lifecycle hooks are not correctly implemented.

Common Symptoms

• Multiple charts stacking over each other inside the same canvas.
• Memory bloat and high CPU usage after navigating between chart pages.
• Charts not updating or duplicating data despite state changes.
• Canvas not being cleared upon component unmount or rerender.

Root Causes

• Improper destruction of Chart instances on component teardown.
• Using the same canvas element without resetting the Chart context.
• Failure to assign or track Chart instance via refs or stateful variables.
• Double instantiation caused by useEffect/useLayoutEffect timing issues.

Diagnosing the Issue

Step 1: Inspect DOM Nodes

Use browser dev tools to monitor canvas elements. Ensure no duplicate canvases exist post-navigation.

Step 2: Check Active Chart Instances

Chart.js exposes all active charts via `Chart.instances`. Use this in dev tools to detect memory leaks:

console.log(Object.keys(Chart.instances));

Step 3: Enable Debug Logs

Wrap Chart creation in a logging utility to observe re-instantiations:

console.log("Creating Chart instance");
new Chart(ctx, config);

Step-by-Step Fixes

1. Always Destroy Old Chart Instances

Use component lifecycle methods to explicitly destroy charts before creating new ones.

useEffect(() => {
  const chart = new Chart(ref.current, config);
  return () => chart.destroy();
}, [config]);

2. Avoid Implicit React Renders

Prevent charts from rendering twice by using `useRef` and guarding `useEffect` dependencies correctly.

3. Use Unique Keys for Chart Components

Assign unique keys to chart containers if used within list renders or dynamic mounts to prevent stale instances.

4. Wrap in a Chart Manager Utility

Create a chart lifecycle utility to manage creation, update, and teardown cleanly across multiple components.

class ChartManager {
  constructor() { this.instance = null; }
  init(ctx, config) {
    if (this.instance) this.instance.destroy();
    this.instance = new Chart(ctx, config);
  }
  destroy() {
    if (this.instance) this.instance.destroy();
  }
}

5. Handle Resize and Navigation Events

Use Chart.js's responsive options carefully and unbind any global listeners on teardown.

Best Practices

• Use `responsive: true` but validate that parent containers resize correctly.
• Don't mutate chart data directly. Always call `chart.update()` after data change.
• Use a framework-agnostic wrapper library or build one that respects component boundaries.
• Perform load testing on dashboard pages to observe chart-related memory consumption over time.

Conclusion

Chart.js offers excellent flexibility and performance, but its imperative rendering model clashes with declarative UI frameworks when not managed properly. Ghost charts, memory leaks, and DOM persistence issues arise from improper lifecycle management. By explicitly managing chart creation and destruction, using ref tracking, and enforcing clean teardown patterns, teams can ensure stable, maintainable chart integrations across dynamic enterprise-grade applications.

FAQs

1. Why does my Chart.js chart render twice in React?

This typically happens due to multiple `useEffect` calls or missing dependency guards. Ensure charts are not initialized more than once per mount cycle.

2. How do I remove ghost charts after navigation?

Call `chart.destroy()` inside `useEffect` cleanup or equivalent lifecycle method to remove canvas bindings and free memory.

3. Can I reuse canvas elements across charts?

Yes, but you must fully destroy the previous chart instance before reusing the canvas to avoid overdraw or incorrect rendering.

4. What's the best way to debug Chart.js memory leaks?

Track `Chart.instances`, use Chrome dev tools for heap snapshots, and monitor lingering DOM elements or listeners.

5. Is there a framework-specific wrapper for Chart.js?

Yes. Libraries like `react-chartjs-2` provide declarative wrappers, but still require manual cleanup and lifecycle awareness.