Here I am just a little confused about how a dip in the off axis relates to a the bump in the directivity line?
As you get more involved with the "math", it will seem clearer.
In the beginning, as so often, it is easier to just accept the context.
Maybe a mnemonic will help you to get a rough idea when evaluating a CTA-2034 diagram - it's a little silly, but maybe it makes a
very rough evaluation of the diagram easier for you.
To evaluate the CTA-2034 diagrams, I first look for anomalies. Very roughly and simplified one can say that humps generally influence the sound of a loudspeaker more negatively than dips (as others have said) and an "imaginary" mirror line runs through the diagram.
So we draw a "path" in the CTA-2034 diagram and let a cat with a hump
*** and its mirror-image equally evil/bad twin wander through the diagram.
All frequency ranges that show a hump or a transition from dip to hump (or vice versa) should be viewed critically, as they can have a negative effect on the sound.
For all curves that represent a directivity index (SPDI, ERDI, hor ERDI, ver ERDI,...), the mirror-image equally evil/bad twin comes into play and we look for dips.
*** What kind of association you have with the "evil/bad" hump is of course irrelevant. It can be a dromedary that bit you as a child or the (sorry, not PC) "The Hunchback of Notre-Dame" (tragic, not "evil/bad", but you know what I mean....).