The Congressional Budget Office has published Sources of the Growth and Decline in Individual Income Tax Revenues Since 1994:

Federal individual income tax revenues have risen and fallen by significant amounts since 1994. Although income tax revenues generally rise and fall with the economy, what is exceptional about the period since 1994 is the dramatic change in revenues relative to the nation’s GDP. The ratio of individual income tax revenues to GDP reached an all-time high in 2000, followed by a 50-year low in 2004. This CBO paper explores the forces that have led to the rise and fall of individual income tax revenues relative to total economic output since 1994.