Secondary y-axis? Where do I add the data points as the 0 is on the other side?

Not clear what you're asking, or what you are trying to achieve?

Is this Excel related?

No, nothing excel related.

I don't understand what a secondary y-axis is and how would I plot the data for it?

A secondary y-axis is the same as used in Excel.

Normally you have a y-axis on the left hand side of your graph, and when plotting points you determine the position of the point by using that scale, on the left.

If you had a second set of data, whose values were radically different to the first set, but you wanted to plot that data on the same graph to compare the two, then you use a secondary y-axis, which would go on the right hand side and you'd use that scale to plot the points for your second set of data.

Both sets of data would have the same x-axis.

I understand where to put the secondary y-axis now but I still don't understand how I plot the data for it?
a)hypothetically reversing the x-axis so that '0' is also on the right hand side
b) using the x-axis the same way I plotted data for the first y-axis.

The scale/values assigned to the the x axis don't change, so option b)

Thanks so much

See I'm clumsy, look at my response in a), how bad at maths am I?!