Samples in a PCA form lines, what are the reasons? I have come across some PCA plots in which the samples are forming "lines", such as this one:



Or this one:



What kind of data can generate such PCA plots? Are there anything wrong with these PCA?
In general (or common cases) you would find those lines when applying PCA, which are the result of finding those Principal Components Analysis, meaning the directions of maximum variance of your original datatset, i.e., the components which give you more information out of your data.
As explained with examples in scikit-learn docu: PCA finds orthogonal directions in the raw feature space that correspond to directions accounting for maximum variance.