Methods to estimate demand curves

Kotler in one of his works cited as an example of the demand curve for motor oil.

I will use this curve to illustrate my statements. Nevertheless, for the convenience of the curve was slightly changed. Firstly, presentation of the demand curve will be correct from a mathematical point of view. Price is applied to the x-axis, and the volume of demand - on the y-axis. Secondly, the curve is smoothed in the prices 53 cents. These changes do not alter the fundamental nature of demand depends on price. The result is shown in the picture.

 

Pricing.