I would imagine this is happening because the y axis adjusts dynamically to fit all the data points, unless you set explicits overrides. As you are zooming out you are likely catching a higher value, and so the graph adjusts to draw it
Depending in use case you could add another graph where you do a two step aggregate window. The first does the mean for every , 10s in my example. Then an max for v.windowPeriod. Note that this gets rather CPU intensive when you do a longer time span. For me the time out threshold seems to be about a week.