Here's what the site does behind the scenes:

    real_price($ASSET, t) = nominal_price($ASSET, t) * (price_level($NOW) / price_level(t))

Where price_level(t) is the CPI-U series (with interpolation).

If t = $NOW, at the right side of the chart, then the fraction goes to 1, so that

    real_price($ASSET, $NOW) = nominal_price($ASSET, $NOW)

(Though many comments are requesting alternative normalization schemes!)

For cash (USDOLLAR), nominal_price(USDOLLAR, t) = 1 for all t -- the nominal price of a dollar bill is always 1. So https://totalrealreturns.com/s/USDOLLAR is plotting a curve that looks like 1/price_level(t). (Actually it's 1*price_level(now)/price_level(t), because of the normalization above.) And you can download price_level(t) from https://download.bls.gov/pub/time.series/cu/.