Line chart zoom sync bug

**Describe the bug**
If one line chart is zoomed in and then the other zoomed out, the second syncing does not happen despite the current callback watching for changes in plot properties.

**To Reproduce**
Start the dashboard.
1. Zoom in the first chart (the two plots will sync still).
2. Zoom out on the second chart.
3. See error: now the two plots are out of sync.

**Expected behavior**
The syncing callback should keep the two line chart zoom setting in sync at all times regardless of the order of zooming.

**Additional context**
Look at `app_components.py` and the `update_zoom()` callback.

Open app_components.py and inspect the update_zoom() callback. Start the dashboard and reproduce the issue by zooming the first line chart in, then zooming the second out. Done means both line charts remain zoomed to the same setting regardless of which chart is changed first.

Written by the indexing model from the issue text.