Interpretation a. Each data point is plotted on the probability plot, which has the actual data units on the horizontal axis. The vertical axis represents the estimated cumulative probability. If the distribution is normally distributed, the data points will form a straight line on the probability plot. b. Data points on a probability plot will never be perfectly smooth, even if the data are Normal. Minitab places 95% confidence interval limits on the diagram (see the lines next to the plots). If all the points fall within the lines, you can assume the data are normally distributed. Extreme data points are more likely to be outside the 95% limits.