[…] You can use the scree plot as a graphical tool to help you choose how many PCs to retain. In the scree plot for the iris data, you can see (on the "Variance Explained" plot) that the first two eigenvalues explain about 96% of the variance in the four-dimensional data. This suggests that you should retain the first two PCs, and that a projection of the data onto the first to PCs will give you a good way to visualize the data in a low-dimensional linear subspace. …