Replace strict `==` with `all.equal()` and move the check into a
function defined in the plot object.

Use constant ylim for multi.panel with different y-axis ylim. The main
panel ylim are recalculated using the range of all columns, even when
`multi.panel = TRUE` and only one column is plotted on the panel.

Closes #368.

Co-authored-by: Joshua Ulrich <josh.m.ulrich@gmail.com>