The linear line (dotted line) is a line fit to the Liquid STEEM to show if Liquid STEEM is going up or down over the entire time period.
STEEM is created at a high rate. Currently 8.85% per year. Many Steemians have a concern about STEEM inflation with the high rate of new currency creation. This graph shows since February, more STEEM has been converted to SP than new STEEM creation. This means that even with the high rate of new STEEM creation, the available supply has decreased.
For a breakdown of how Steemit Rewards system works:
https://steemit.com/steemit/@socky/steemit-rewards-breakdown-a-simplified-explanation-with-illustrations
https://steemit.com/life/@socky/do-you-understand-your-steemit-keys