The Iowa caucuses, the first contest in the Democratic primary season, are happening Monday.

Income and wealth inequality have been central to several candidates' campaigns.

We made six charts that illustrate why it's such an important topic.

Namely, inequality has risen in the US over the last several decades.

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The Democratic Iowa caucuses are happening on Monday, and inequality has been one of the biggest issues in the nascent presidential campaign.

Among the frontrunners in the race, Senators Bernie Sanders and Elizabeth Warren have made inequality and related issues central to their campaigns. Both candidates support drastically raising taxes on the wealthiest Americans, along with programs like a Medicare for All single-payer health system and student-debt forgiveness aiming to improve the fortunes of the middle- and working-classes.

Other candidates have taken different approaches to inequality. Businessman Andrew Yang has proposed a $1,000 per month "Freedom Dividend" as a form of universal basic income, seeking to directly increase Americans' incomes.

To help see why income and wealth inequality are such perennial issues, we made the following charts to illustrate how the fortunes of the top 1% have outpaced those of the rest of Americans.