On occasion I have experienced a similar problem plotting time series in the multiple minigraphs format. The dates overprint themselves on the X axes.
It might be better if the X axis could not only be rescaled, but in addition the data plot interval values increased eg instead of 10 intervals, reduce to 5 ie every 2 years instead of every year.
I recognise this might be tricky in the multiple plot format, but I imagine ought perhaps (not being a programmer} to be more straightforward to resolve for a single graph.
Brian