I have solved the issue of the tick marks using FrameTicks

FrameTicks -> {{{0, 2, 4, 6, 8, 10, 12, 14}, 
   None}, {{0, 5, 10, 15, 20}, None}}

I will raise a new discussion regarding adding a legend when using CombinePlots. Thank you for your help Daniel.

Thank you. The second of those seems to be the more appropriate and I have been experimenting with that approach. Most things are working fine but I have two issues with the result.

One is that I can't seem to get the y-axis major tick marks and labels where I want them (and where I think they should be). The range of the y-axis is 0-14 and I want the major tick marks and labels to be at intervals of 2 but in fact they are at 0, 2.5, 5.0, 7.5, 10.0 and 12.5.

The other issue is that I am using this to place a legend:

PlotLegends -> 
    Placed[LineLegend[{legend1, legend2}, 
      LegendFunction -> (Framed[#, FrameMargins -> -2, 
          Background -> White, FrameStyle -> Directive[Black]] &), 
      LegendLayout -> "Row", LegendMarkerSize -> {{8, 8}}, 
      LabelStyle -> {FontSize -> 10}], {Right, Bottom}]

and this is not working at all (exactly the same code is used to place a legend on a different plot elsewhere in the notebook and it works fine). I am attaching the notebook and a file containing some required data. The relevant code is in a function named makePlot2.

Elsewhere in the same notebook I have another plot with two sets of data and I am using this approach:

ListLinePlot[{Reverse[dataPairs1, 2], Reverse[dataPairs2, 2]}, 
 Frame -> True, lots more options]

and that works really well so I was hoping to do something very similar again but, as I said in my original post, I cannot seem to get it to work,

Attachments:

Dissociation con...txt

V18 Calculation_...nb