A couple of work around options neither of which are ideal.
You could hide the x axis in the chart and then layer text boxes which could be colour coded over the top of the chart. However the alignment will probably get thrown out depending on selections made. Another option if you only have a small number of values on the a axis would be to layer an individual chart for each value one on top of the other and colour code the axis on each chart, not forgetting to make each additional chart transparent. Again not a solution I would be happy with but just thought I'd throw it into the mix. I wouldn't disagree with Gysbert though and probably better to look for another solution.