You can use the Functions® Foreground curve option to "page through" the curves in your graphs. Each time you select this option, the next curve in the series is displayed in the foreground.
This option is only relevant when you have multiple curves that overlap each other. A graph can contain up to 16 curves, some of which may hide important sections of others. This happens especially when some of the curves are formatted as having Area or Stepped Area curve types (in the Curve Attributes menu).
To display a particular curve, select Functions ® Foreground curve repeatedly until the desired curve stands in the foreground.