That's just the "spread" of US govt bonds (of different maturities, ie 3 months period and so on). Not trend of yield.
Guiding principle, is longer you lock up your money... diff yield rates. Yield is coupon (interest) rate over bond price. If bond market valuation goes up, your yield (simply put, ratio) correspondingly goes down -as in 4.5% example. Then you have capital gain on bond price