Plotting a calibration curve for Gas-Liquid Chromatography Question:

The given graph of abundance of different samples of butanol with respect to time is-


Textbook solution:



But why aren't the points like this:
$(20,5);(40,10);(60,15);(a,7)$
after which, to plot the curve we can use $y=mx+c$ and then find the value of $a$?
If I remember correctly, the amount of your compound isn't proportional to the peak height but to the peak area.

Back in the days, when plotters were just plotters (=no numerical integration of the peaks), people used to cut out the curves and weighted the paper pieces on a balance to determine the peak area.