The Exposure Histogram displays exposure information for a living picture. In Capture Mode, the exposure histogram displays information for the live view and potential shot. In Playback Mode, the exposure histogram displays information for the selected living picture being displayed.
To view an exposure histogram, tap the Exposure Histogram icon in the Menu Bar at the right-hand edge of the screen. The icon will illuminate, and the histogram will show in the lower left-hand corner of the screen. To close the histogram, tap the icon again.
More information can be found in the User Manual, available here.