Absolute: log2(TPM + 1), or the VST value for VST experiments, averaged over each group's samples (wood pictures show each section on its own), white to red. Relative: that value minus the mean over all groups or sections, blue (below) to red (above). Each gene is scaled to its own largest value unless a fixed maximum is given. Relative is not drawn for a gene whose group means are all below the expression floor.