The graph is based on the last 200 spots of W6IWI from the Reverse Beacon Network. Since a particular transmission can be picked up by a lot of RBN receivers, each graph has a limited number of data points. Also, since there may be several hours (or even days) between transmissions, the data is severely undersampled. The chart has a straight line between sample points, even if days apart. This straight line does not indicate a monotonic change in SNR. Times are "browser time" (your local time zone).