Better explanation for the axis gap

In the plot documentation explain what the concentrations left of the gap really are and how they can be calculated from the model.

Maybe do one of these:

• Export adjust_smooth_concentrations so the users can calculate the concentrations themselves.
• Add a column with the "plot concentrations" to the curves data frame. Values where use_for_plotting is FALSE would be NA then. In this case the plotting functions wouldn't need to calculate those concentrations themselves.