This is because the two axes have different number of ticks (the axis with PlotMode = OnTicks has one less tick then the one with BetweenTicks). The workaround for this is to add one additional item to the invisible series. I have attached a modified version of the project that shows this. Probably you will need to further adjust the converter for your scenario.
Actually I had included this solution the first time but then I went back and forth on a couple of things in order to streamline the solution I forgot to include it in the final project, so please excuse me for the omission.
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