The number of Americans without health insurance declined to 9.1 percent last year, according to federal data released Tuesday. A set of maps released by the Census Bureau suggests an obvious way to decrease the uninsured rate even more: expand Medicaid in the 19 states that haven't.

The Affordable Care Act, commonly known as Obamacare, originally called for an expansion of Medicaid eligibility to people who make up to 138 percent of the poverty level. The Supreme Court ruled that mandating the expansion was unconstitutional, allowing states to opt out. That has left a "coverage gap" in 19 states, where poor people are not eligible for Medicaid, but also do not qualify for the subsidies for private health insurance through the Obamacare marketplaces.

Here's how the map showing how Medicaid expansion looks today:

In the new Census analysis, the researchers compared states that had expanded Medicaid as of Jan. 1, 2015 to those that hadn't, and depicted two portraits of uninsurance in America. The darker blue a state is shaded, the greater the uninsurance rate. It is quite clear from the two maps below, the non-expansion states tend to have higher rates of uninsurance.

"We see much larger drops in the uninsured rates, particularly between low and moderate income people" in states that expanded Medicaid, said Sara R. Collins, a vice president at the Commonwealth Fund.

The report notes that the uninsurance rate decreased among poor people in both kinds of states. The drops in the uninsured rate were bigger for people who made below the poverty level or up to 399 percent of the poverty level in states that expanded Medicaid. Not all the uninsured in each state would qualify for insurance, but a Kaiser Family Foundation analysis, based on last year's data, found that 19 percent of the population of those states that haven't expanded Medicaid -- close to 3 million people -- fell in the coverage gap and would be eligible if Medicaid were expanded.