Currently axis lines and spans report their limits like this:

https://github.com/swharden/ScottPlot/blob/2fa71858df96154e6929ebb4b0db84edfb9a1050/src/ScottPlot/plottables/PlottableAxLine.cs#L69-L72

... but I think reporting real limits (using double.NaN for the unused axes) is best here. Otherwise we have to if elsewhere ...