The function is a wrapper for quantmod::getSymbols().
DGS1MO: 1-Month Treasury Constant Maturity Rate
DGS3MO: 3-Month Treasury Constant Maturity Rate
DGS6MO: 6-Month Treasury Constant Maturity Rate
DGS1: 1-Year Treasury Constant Maturity Rate
DGS2: 2-Year Treasury Constant Maturity Rate
DGS3: 3-Year Treasury Constant Maturity Rate
DGS5: 5-Year Treasury Constant Maturity Rate
DGS7: 7-Year Treasury Constant Maturity Rate
DGS10: 10-Year Treasury Constant Maturity Rate
DGS20: 20-Year Treasury Constant Maturity Rate
DGS30: 30-Year Treasury Constant Maturity Rate