Fetch any of FRED's ~800,000 economic time series by series ID — GDP (GDPC1), CPI (CPIAUCSL), unemployment (UNRATE), fed funds (DFF), and so on. Supports date windowing and the standard FRED units transformations (lin, chg, ch1, pch, pc1, pca, cca, log). ?seriesId=GDPC1&startDate=2018-01-01&endDate=2023-12-31&units=pc1