// Developed by Marc Chaikin, measures the amount of over a specific period.
// forms the basis for the Line. Instead of a cumulative total of
// , simply sums for a specific look-back period, typically
// 20 or 21 days. The resulting indicator fluctuates above/below the zero line just like an oscillator. Chartists
// weigh the balance of buying or selling pressure with the absolute level of . Chartists can
// also look for crosses above or below the zero line to identify changes on .