Add blog post analyzing California billionaire tax marginal rates (#2779)

Analyzes the 2026 Billionaire Tax Act's phase-in structure and marginal rate dynamics. The 5% tax creates marginal rates between 50-60% on wealth in the $1.0-$1.1B range due to the tax applying to total wealth rather than only wealth above the threshold.

Includes four interactive Plotly charts showing:
- Stepwise rate structure in phase-in zone
- Total tax liability by wealth
- Marginal tax rates at each $2M increment
- Comparison of average vs marginal rates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: MaxGhenis

public/charts/california-billionaire-tax-marginal-rates/liability.html

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+On October 21, 2025, labor union SEIU United Healthcare Workers West submitted the ["2026 Billionaire Tax Act"](https://oag.ca.gov/system/files/initiatives/pdfs/25-0024%20%28Billionaire%20Tax%20%29.pdf) (California Initiative 25-0024) to California Attorney General Rob Bonta for title and summary preparation. The initiative would impose a one-time 5% tax on the net worth as of December 31, 2026 of California residents with wealth exceeding $1 billion.
+
+The tax structure creates marginal tax rates on wealth between 50% and 60% during the $1.0-$1.1 billion phase-in range. This occurs because the tax applies to total wealth rather than only wealth exceeding $1 billion, causing rate increases to affect all accumulated wealth.
+
+## The phase-in mechanism
+
+The statutory text specifies:
+
+> For individuals and trusts on whom tax is imposed... the tax imposed is 5 percent of the net worth of such individual or trust. In the case of an individual (other than a trust) having net worth less than $1.1 billion ($1,100,000,000), the tax imposed by this Section shall be reduced by 0.1 percentage point (but not below zero) for each $2 million ($2,000,000) by which such person's net worth falls below $1.1 billion ($1,100,000,000).
+
+<iframe src="/charts/california-billionaire-tax-marginal-rates/phase-in.html" width="100%" height="650" frameborder="0"></iframe>
+
+This creates a stepwise rate schedule. At exactly $1.0 billion, the tax rate is 0% and total tax is $0. At $1.1 billion, the rate reaches 5% and total tax is $55 million. Taxpayers can spread payments over five years, though the liability remains based on wealth as of December 31, 2026 regardless of subsequent wealth changes.
+
+## Total tax liability
+
+Multiplying wealth by the tax rate yields total tax liability. For example, at $1.1 billion, the 5% rate produces $55 million in tax ($1.1B × 5% = $55M). At $2.0 billion, the tax is $100 million ($2.0B × 5% = $100M). The chart below shows the full tax liability schedule.
+
+<iframe src="/charts/california-billionaire-tax-marginal-rates/liability.html" width="100%" height="650" frameborder="0"></iframe>
+
+## Marginal tax rates
+
+The marginal tax rate measures how much additional tax results from a wealth increase. It equals the change in tax divided by the change in wealth.
+
+Because the tax applies to total wealth rather than only wealth above $1 billion, each 0.1 percentage point rate increase applies to all accumulated wealth, creating marginal rates between 50% and 60% in the phase-in range.
+
+| Initial wealth | Initial rate | Initial tax | New wealth | New rate | New tax | Change in wealth | Change in tax | Marginal rate |
+| -------------- | ------------ | ----------- | ---------- | -------- | ------- | ---------------- | ------------- | ------------- |
+| $1.000B        | 0.0%         | $0M         | $1.1B      | 5.0%     | $55.0M  | $100M            | $55.0M        | 55%           |
+| $1.000B        | 0.0%         | $0M         | $1.002B    | 0.1%     | $1.0M   | $2M              | $1.0M         | 50%           |
+| $1.098B        | 4.9%         | $53.8M      | $1.100B    | 5.0%     | $55.0M  | $2M              | $1.2M         | 60%           |
+| $1.100B        | 5.0%         | $55.0M      | $1.102B    | 5.0%     | $55.1M  | $2M              | $0.1M         | 5%            |
+
+<iframe src="/charts/california-billionaire-tax-marginal-rates/mtr.html" width="100%" height="650" frameborder="0"></iframe>
+
+The marginal rate increases from 50% to 60% across the 50 discrete $2 million increments in the phase-in range, averaging 55%. Above $1.1 billion, the marginal rate drops to 5%.
+
+The initiative requires [874,641 valid signatures](https://ballotpedia.org/California_2026_ballot_propositions) by June 25, 2026 to qualify for the November 2026 ballot.

public/charts/california-billionaire-tax-marginal-rates/mtr.html

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 [
+  {
+    "title": "Marginal tax rates in California's proposed billionaire tax",
+    "description": "The 5% tax creates marginal tax rates between 50% and 60% on wealth between $1 billion and $1.1 billion.",
+    "date": "2025-10-25",
+    "tags": ["us"],
+    "filename": "california-billionaire-tax-marginal-rates.md",
+    "image": "california-billionaire-tax-marginal-rates.jpg",
+    "authors": ["max-ghenis"]
+  },
   {
     "title": "Comparing PolicyEngine and official UK poverty rates",
     "description": "We compare PolicyEngine poverty estimates with official DWP statistics and explain the methodological factors behind the differences.",