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. Left drag over an area to to zoom to that area and right click to go back to the full graph. Times are "browser time" (your local time zone).