I was dealing with issue 271. From elenna's alog and summary pages of these days, from Sep 05 to doday, we can see that these 20 Hz blue short lines mean disappearing of noise in this frequency. From this website, I have checked many devices, where lots of same blue short lines appeared, which means that the noise in these devices have a strong coherence. From this page, we can see history of these devices, I found that this blue short line occured on half of the days of each month in September and October. But on Nov 1 and Nov 2, it became 15 Hz blue dashed line, rather than 20 Hz blue short line. But since then, this blue short line phenomenon has not appeared again. As for the souce, I guess it was the air conditioner turning on and off or other equipment that causes the blue short lines. Nowadays nothing similar is occurring.