Disabled chart add-on displays still attempted to be attached

When a chart add-on display is present in a View, but marked as disabled, the get_children_chart_displays() function still attempts to load it, but as it is disabled, zero results are returned. This is one of the scenarios that would result in the error "All series on a given axis must be of the same data type" appearing (see #2052429 for others).

The attached patch fixes this issue.