Contour plot: A topographical map drawn from data comprising observations on the three variables. One variable is represented on horizontal axis and the second variable is represented on the vertical axis. The third variable is represented by the isolines. These plots are many times helpful in data analysis, especially when the searching for maxima or minima in such type of data. The plots are most often taken in use to display the graphically bivariate distributions in which case the third variable is value of the probability density function corresponding to the values of the two variables. An alternative method or technique of displaying the same material is given by the perspective plot in which the values on third variable are represented by the series of lines constructed to give the three-dimensional view of data. The figure 43 drawn above gives examples of these plots using the birth and death rate data from the number of nations with a kernel density estimator used to calculate or measure the bivariate distribution.