I'm wondering about the opposite of a plateau. I mean, I tend to associate "valley" with a downward spike, as in peaks and valleys. I want to talk of "a flat bottom on the data graph" in the same way that you might talk about a plateau in the graph.
Trough might work. It is the low point between peaks or waves.
It also has an economic definition.
In general, the business cycle is said to go through expansion, then the peak, followed by contraction, and then it finally bottoms out with the trough.
bowl: a bowl-shaped depression of the land surface.
A flat-bottomed bowl (or basin) is a pan.
pan (also called "playa" or "dry lake"): flat-bottomed depression that is periodically covered by water.
If I were describing a flat on the graph that was down near the axis, I would call it a low plateau. A plateau doesn't necessarily imply height beyond being above baseline.
If it on the baseline, I would call it either a baseline plateau, or even just name the axis of the graph itself. (The x-axis, etc.)
If it is a true low-point, you can use the term nadir.
I think the answer is just valley. When I think of a valley I think of something that goes down, is then pretty flat, and goes back up. If that is what your graph does I would go with valley. If it is sharper then you could use gorge or canyon.
Other graph phrases:
- stagnant dip
- dip then flat line
If you don't think valleys are flat you could go with inverted plateau.
Since you're coming at this from the perspective of statistics, why not the mathematical concept of "asymptote"?