while reading about line following robots i came across these wonderful article on line following robots


while reading this articles i could not understand following thing in the article

Signal processing of line detection

Right image shows the actual line posisiton vs detected line position in center value of 640. The microcontroller scans six sensors and calcurates the line position by output ratio of two sensors near the line. Thus the line position can be detected lineary with only six sensors
also i could not understand the graph given on the right side

if someone could help me understand this techniques specially how he calculates the line position by output ratios of the two sensors near the line linearly. and also the graph and how it is plotted