Fig. 4. Correlation of two assays (x and y) across three studies (color and shape) shows a weaker response on the y-axis for the study shown with magenta ‘x’ symbols. A separate area for values ≤ zero appears when needed on log scales (a). Gutter plots with undefined values for x or y appear when comparing two variables with only partial matches (b). The study shown with green ‘+’ symbols probably tested the two assays on samples from mutually exclusive times.