This is useful if you have previously deleted the area of the chart containing the Volumes and now want to enter them again.
Normally the Volumes are displayed with a Histogram chart that you can setup by selecting the graded curve and typing the letter "I" of the keyboard, or with the menu that appears by right-clicking on the area of the chart, selecting, under "chart Type", the option "Histogram". The same menu appears by selecting the "Config" button, located in the lower right corner of the chart window.