Uncertainty display. The orange density contours in the three panels represent an arbitrary bivariate PDF of two variables x and y. Panel a: corresponding traditional error bar: the dot is the mean and the bars show the ±1σ extent along both axes. Panel b: widely-used uncertainty ellipse, which can be viewed as the mode and 1σ contour of a bivariate normal distribution having the same means, standard-deviations and correlation coefficient as the PDF. Panel c: concept of SUE introduced in Appendix F.1.