Population growth (or loss) is not distributed uniformly among the states.
The line shows, for each state, the difference between the state and the national population change. It is cumulative from the first year, which is 2000 in this example. A flat line that is very close to zero does not mean that a state is not growing. It means, instead, that the state’s population change is close to the rate of the nation as a whole. Montana and Minnesota are examples.
A line that is falling below zero does not mean that a state is losing population. It may mean that the state is not growing as fast as the nation. Kansas is such a case, with a slowly growing population.
The source of this data is the United States Census Bureau population estimates, along with my calculations and charting.