Long pressing the Chords Tab will unfolde three menus. Then are Chords Reverse Search, Chords by note andf Chords by keys. Choose By Key to enter Chords By Key Mode.
The interface of Chords By Key is similar with Chords By Note interface, however, the top area of the screen displays different elements.
By Note displays chords by their root, while By Key displays note by the Key/Tonic. Learn more information in Wikipedia:
 Tap b and # button can change keys. Button b represents a semitone down while button # represents a semitone up.
 Tap the Title (C Major/A minor) to open the Key Signature Setting Interface. You can change to your desired key quickly as the image shows above.
 Chords displayed on the main screen are arranged based on Circle of 5th, and they are categorized into Basic Chords, 6 Chords, 7th Chords etc.
Take the image above for example, We’ve chosen key of C Major/A Minor. In basic section, first line shows IV, I and V chords, while the second line shows ii, vi and iii chords. They are F, C, G, Dm, Am and Em, correspondingly, just the same with the arrangement on the Circle of 5th.