OnSong has the ability to detect chords in text-based chord charts and then display those chords with different formatting options. You can also have OnSong lookup chord diagrams in a chord library and draw those diagrams on the page. You can do this by turning on chord diagrams on the Chords Tab of the Style Preferences Menu. You can learn more about this and other features in the Style Preferences Tutorial Video.
Keep in mind that you can tap on these chords and diagrams to show the Chord Inspector to see different variations of chords. You can also edit your Chord Diagram Library. You can learn more about chords and diagrams in the Chords Video Tutorial.
When you enable chord diagrams, this will show all chord diagrams present in the song by default. To change this, tap on the gear icon in the Menubar to open the Utilities Menu and then choose Settings. Tap on Display Settings on the left and then choose Song Formatting on the right. Next, locate the Chord Diagrams section and the Show Only Preferred Chords and turn that option on. When you tap Done, you chord chart will likely show no diagrams at all. The reason is you need to select the diagrams you want to display. To do this, tap on the chord letter to open the Chord Inspector again. You can then tap on the chord diagram in the Chord Inspector to select it. Once it is selected and surrounded in a grey background, close the Chord Inspector by tapping elsewhere. The song will refresh with only the selected chord diagram will be displayed. Repeat this for each chord diagram you want to appear.