I have an XT1 and I guess the later models will be similar
To see the histogram on the back screen you need to press Menu and go to the Tools area
Select Screen Set-Up
Toggle Across and Down until you get to Display Custom Settings
Toggle Across and Down until you get to Histogram and Select it. Then Exit via the DISP BACK button.
Turn the camera off and back on. Not sure if this is necessary to do this step.
Now press the DISP BACK button again and turn the front wheel to select C for custom on the tool bar on the bottom of the screen.
You now have your custom settings on the back screen such as Grid Lines, Electronic horizon and Histogram etc