Yeah, boof! We are off and running.
Mac - 'Index' is tabulated for unit loads at various positions and based on an arbitrary datum such as the nose or a particular other fuselage position. The 'fanned lines' indicate the % position of the C of G (normally based on the wing leading edge and bizarrely known as 'MAC'
) from which a tailplane trim for take-off is derived (from yet another look-up table - which again varies with weight!!??). Let's call it 'Trim'. (I've often pondered why aviation does it this complicated way!)
With the advent of 'computers' both in load offices and the cockpit these 'old fashioned' sheets are becoming mostly extinct, but believe it or not they were a very quick ('back of a fag packet') and accurate way to establish a/c load and balance - once someone else has done all the line drawing/equation crunching for the basic chart of course.