The Marked Zone feature allows you to place a rectangle inside the ChartArea. This allows you to mark a specific part of it, in order to easily look at all the data that belongs to this area. In order to creat a Marked Zone you have to use the MarkedZone class and add an instance of it to the Annotations collection of the ChartArea. You also have to set its StartY and EndY properties for a rectangle that spans along the entire X-Axis, or the StartX and EndX properties for a rectangle that spans along the entire Y-Axis, or set the four properties at once in order to create a rectangle that is limited in both directions.