This resource graphically details the value history of the current performance counter, in this case, the total amount of received mail for the selected mailbox. Hovering over any time period within the chart displays detailed information about that time period in the tooltip.
The colors of the main chart correspond to the labels in the legend. You can choose which data to display by using the check boxes in the legend.
You can have the chart show a predetermined time period of data by clicking on any one of the three Zoom buttons in the head of the Zoom column. Alternatively, you can have the chart show a specific date range by dragging the mouse over an area of the chart or manipulating the sliders of the lower chart.