I'm still not comfortable with this method returning a mutable container (with a nested structure) from this method. Previously it returned a tuple of matplotlib artists which seemed reasonable. Now it returns the instance's 'artist cache'. @rhattersley and I feel that the gridliner add/draw process is overly complicated, and the means of allowing access to the artists needs overhauling. In the meantime it will be cleaner if you remove this attribute altogether. Providing better user code access to the line/labels can be addressed when this Gridliner class is refactored (possibly as a matplotlib artist).