pre_text: ANG's revenues are principally derived from sales of compressed natural gas. ANG recognizes revenue from the sale of natural gas fuel primarily at the time the fuel is dispensed. In December 2019, the U.S. Congress passed an alternative fuel tax credit ("AFTC") which will continue to support the use of natural gas. The AFTC is retroactive beginning January 2018 and extends through 2020. The legislation extends the $0.50 per gallon fuel credit/payment for the use of natural gas as a transportation fuel, and the Alternative Fuel Vehicle Refueling Property Credit, which extends the 30 percent/$30,000 investment tax credit for alternative vehicle refueling property. Net revenue after customer rebates for such credits recognized in 2019 was $10.6 million. As a result of the Bipartisan Budget Act of 2018, signed into law on February 9, 2018, all AFTC revenue for vehicle fuel ANG sold in 2017 was collected in the second quarter of 2018. Net revenue after customer rebates for such credits recognized in 2018 was $2.6 million. Disaggregation of Revenues The following table disaggregates ANG's revenue by type (in millions):
table: Years Ended December 31,
table: 2019 2018
table: Volume-related $ 27.5 $ 16.5
table: Maintenance services 0.1 0.1
table: Total revenue from contracts with customers 27.6 16.6
table: RNG incentives 0.5 1.3
table: Alternative fuel tax credit 10.6 2.6
table: Other revenue 0.3 0.2
table: Total Energy segment revenue $ 39.0 $ 20.7
question: Analyse this data from a financial earnings document. What was the increase / (decrease) in the volume-related revenue from 2018 to 2019?