Hey guys, a data viz noob here so excuse my foolish question.

Say we have a chart with bars and lines combined. And it's a dual Y axis chart. One axis is for the bars and the other one for the line. Now my understanding is that the two axes should have some correlation, aren't they? Is there any scenario where it makes sense the axes not to have correlation?

Also any links to reading material as to what are the rules of those type of charts and should the axis be correlated or not, would be appreciated.