Enhance migration EDA visuals with presentation-grade styling and new timing insight charts

[Copilot]

This PR upgrades notebooks/eda_migration_enhanced.ipynb to produce cleaner, more presentation-ready visuals and adds deeper comparative analysis between visa issuances and border encounters. It also adds a report.md documenting the visualization and insight improvements.

• Visualization quality improvements

  • Updated the global plotting palette to a tighter, presentation-friendly scheme for consistency across figures.
  • Kept chart formatting coherent across sections (titles, spines, legends, readability).

• New insight charts for presentation

  • Added 12-month rolling trend comparison to reduce monthly noise and highlight macro movement:
    • data/plots/rolling_12m_visa_vs_encounters.png
  • Added lead-lag correlation profile (-6 to +6 months) to show timing relationship strength:
    • data/plots/lead_lag_correlation_visa_encounters.png

• Notebook structure and flow

  • Added section: ### 2.3 Additional Insight Charts for Presentation
  • Renumbered subsequent heading to keep section sequencing clear (2.4 Identify Surge Events).

• Documentation

  • Added /report.md with a concise breakdown of charting upgrades, added outputs, and analytical value for presentation use.

Example of the new lead-lag analysis logic added to the notebook:

lag_values = list(range(-6, 7))
lag_corrs = []

series_visa = merged_df['visa_issuances'].astype(float)
series_enc = merged_df['Encounter Count'].astype(float)

for lag in lag_values:
    shifted = series_visa.shift(lag)
    valid = shifted.notna() & series_enc.notna()
    corr = shifted[valid].corr(series_enc[valid]) if valid.sum() > 2 else np.nan
    lag_corrs.append(corr)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Data Visualization</issue_title>
<issue_description>Take a look at the EDA notebook made at notebooks/eda_migration_enhanced.ipynb.

1. Make the charts in the notebook even better and prettier, choose better colors and formatting.
2. Try to see its analysis, see if you can make more charts that we can use in our presentation.
3. Add additional plots and charts that offer much more insights into the analysis
4. Create a report.md file detailing your changes and how you have improved the current implementation.

Save all the charts you make. Make a directory plots/ in the data directory. Save all the plots and charts you make there.</issue_description>

<agent_instructions>Take a look at eda_migration.ipynb notebook in the notebooks directory, see if you can improve on it by adding additional plots and visualizations that offer much more insights. Create a report.md file at the end of your implementation detailing the changes you made how you have improved it. Plan your implementation then implement.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Copilot Data Visualization #4

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