Consider appropriate action when aggregating but rebase points aren't the same across all levels of aggregation

[Bergam0t]

At the moment, if rebase points aren't the same at every level of aggregation, it just fails to plot

Would be good to consider what is the most sensible behaviour for this.

• Take any/all rebase points?
• Take no rebase points?
• Refuse to plot and return clearer error message?