Both of your expressions are intelligible. Neither sounds entirely natural to the ears of a native speaker.
The problem is not grammatical, but semantic, and arises because you are using space (i.e. the X axis) to represent an interval of time. This allows you to use both spatial and temporal expressions to describe the picture and what it represents.
You may recall Alfred Korzybski's admonition that the map is not the territory. Your graph is a map. Your verbal descriptions may apply to either the map or the territory, but if you use both it's easy for the reader to get confused or distracted.
Within is a spatial expression, e.g. a thing can lie within an enclosure. It can also be applied to things where time is part of the enclosure, e.g. within Beethoven's First Symphony there are four movements. It's "map-like" in its associations.
During is a temporal expression. You might, for example, imagine yourself listening to the symphony in a concert hall, and that during the first movement you received a phone call, which you answered during the intermission. You wouldn't describe these actions as happening within either the performance or the intermission. For your "signal", during is more "territory-like", i.e. you are talking about "what is really happening" as opposed to pointing out a feature on the map.
For can be used in either way, e.g. I could walk for a mile or for an hour. However, a reader familiar with computer programming will likely associate for with iteration, with its sense of moving along a sequence of events (like footsteps).
If your larger objective is to describe your data to the reader, I would suggest using a simple and consistent style that stays with either the map or the territory. For example, if you use the map (spatial) form, describe first the enclosure and second its properties, e.g.
- In the decade from 1870 to 1880, the M values were high.
- In the interval highlighted in Segment k (1897 to 1913), the M values were low.