From what I understand of WW it follows the zig zag pattern of price movement up or down. The wave histogram is the cumulative volume from the lowest point of the price zigzag (swing) to the highest or vice versa. You change the setting of the height of the price zigzag (swing) (from say .5 to any value above or below – not negative).
This allows you to choose the setting of the zigzag (swing) that is most appropriate for your work. The volume wave will follow the price zigzag (swing) starting from the lowest or highest to next high or low based on the individuals setting, and will continue going higher or lower till there is a reversal in price based on the setting.
This allows you to see the cumulative volume for that move of the price swing. (eg. your setting may be 1.5 for stocks or a value appropriate for currencies or commodities. The wave will continue beyond 1.5 till there is a reversal of that amount or less if you want to provision for that say 1.) Hope this helps.