1) Select the chord line whose diagrams are to be made "permanently visible" by putting the caret in the target line, or else select several lines by hitting Shift+Down Arrow.
2) With the line selected, choose View|Line Appearance|Chord, Tab, and Rhythm Line Appearance...
3) When the dialog appears, check the box for "Chord diagrams in current or selected line(s) should be:", and then choose "Always visible, even when 'hide diagrams' is checked". Hit OK.
To turn off this option for a chord line, select "Hidden when 'hide diagrams' is checked".
Tip: It's handy to use "Split Windows" with chord diagrams visible in the top part of the window and only chord names visible in the bottom. It is even possible to activate scrolling for the bottom half of a split window while leaving the chord diagrams visible in the top half.