You are right.

A tertiary axis certainly would make the chart quite confusing. I know about panel_charts, I have created some in QlikView, using different scales - I do think it's easier and less confusing than using two axes - especially because in Excel, most people who use a secondary axis don't include any hint on which measure to "read" on the left-hand one and which on the right-hand one ...
A chart with a horizontal split would be even better - I'd have the x-axis for the months from Jan to Dec and beside that, a second x-axis with just one datapoint which would read "Total".

I am just wary of offering people tools they are not used to, even if they are in fact better ... that can be tricky. I'll try it out, though, if only for myself.

I think