Interpolating Using a Graph

I have data with 3600 lines. When I plot this data on a graph, a certain part of it presents a behavior that is out of the standard. I would like to know if through the graph, I can indicate which part of the data has this behavior and interpolate this part by the beginning of the data?

[Jacob Wood]

You can try detecting and removing outliers: https://www.mathworks.com/help/matlab/ref/rmoutliers.html