Your question is unclear. You say your picture size is [869 375] but then you tell MATLAB to use columns 1 through 755 of img. Does your image have 375 columns or 755? It's also not clear how 178:198 are relevant: do you want to use those rows of img instead of rows 415:435?
And what do you mean by "average vertical line" -- do you want the line whose data is represented by the center row of your img, do you want the line that is the mean of your data, or do you have some other meaning in mind?
