[iOS] How do I detect legend selection changes to consequently hide or reload annotations?

I am placing annotations at the top of every bar in a stacked bar graph (and a standard bar graph on another screen) showing the total of all the stacked bars by iterating through my data, generating the sum, and placing it at the correct x,y (an incredibly manual process that seems like there should be much easier to do but I found no other way to do it – if there is a better way, I would love to hear it, but that’s a secondary issue and not my current problem).

I also have a legend where the user can select / deselect series.

When the user selects/deselects a series from the legend, I need to hide all my annotations and then recreate them because the sums of the stacked bars has changed (or if no bars are showing, remove my annotation completely). My assumption is I could reiterate through my data and check series to see if they are visible (isVisible) and recalculate the sums and recreate the annotations (another very manual process but I can probably work it out).

My primary problem is I see no way for me to intercept that this legend event occurred. The series get hidden internally and I never have a chance to do anything with the annotations. It seems like this may be doable on other platforms but I’m at a loss on iOS. There is no delegate / block event handler / etc.

Any thoughts?