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That chart has two scales. The blue line is on the scale to the left. The orange line is on a scale to the right. The scales are adjusted so that you can see what is happening with both. This way you can see if they are trending up, down, the same, or different. The sudden drop is when someone takes a bunch of STEEM and stores it as SP. This converts Liquid STEEM to non-liquid STEEM. When you put both on the same scale, you have my second chart.