The Ministry of Finance Japan very generously provides data on JGBs back to 1974. Here is a quick example how to pull it into R and then graph it.
R code in GIST (do raw for copy/paste):
As with most everything in economics, demand is the price setter. Demand for moolah is determined, in the limit, by technological progress, i.e. the real rate of return on physical investment is what creates the demand for moolah. It's no secret that Western corporations have, on the whole, run out of investment opportunities. Some, if you Google it, even admit it. Since "savers" can't eat cash, the return falls to what the physical investments can generate. Right now, it ain't much. Thus the flight to fiduciary investment, where the return is more subject to fiddling by the "system".