Why does it start from March 2018? Is that when the testnet launched? And it just became the mainnet afterwards?
Wondering this question too. However, not found an answer to it so far.
It is simply going down because at the present time GRIN has one of the highest inflation in the market (I don't know if it is the highest); it will gradually decrease over time, until it is almost negligible after 30 years, but the objective fact remains that we have 86400 new Grins on the market every day, more than 31 million Grin every year,
FOREVER.
All this in a relatively stationary / lateral market, with many "old" projects close to the release of Mainet or important partnerships, with "problems" relating to wallets to send and receive Grin ( completely different and more complicated than the previous Coins ), here is we can explain a slight and constant drop in the new currency!
Finally I believe that the "creators" of GRIN did not want the umpteenth coin to speculate and earn above, the umpteenth gold digital to be kept in the wallet without spending it, because tomorrow has a higher price (why spend today $ 1000 if tomorrow can worth $ 5,000 / 10,000?); I believe that the creators of GRIN wanted a new Coin minable, anonymous, fast, and above all with a light blockchain.
So let's say that today GRIN is not a store of value, it is not yet widespread as a means of payment and is not yet user friendly (it could become so in the future); on the other hand GRIN is today a new Coin with
privacy, no address, with a new and innovative protocol (MimbleWimble and Couckoo Cycle), with a light blockchain, medium of exchange!