My mistake. The easiest way I can see, to do this is to create a computed column that multiplies your axis #2 field by -1, plot this field on your second axis and then do what I said above and reverse the positive and negative values in the labels. Let me know you BIRT version if you need an example.
Take a look at this report. I used a scripted dataSet to recreate data similar to yours, so instead of creating a computed column in the dataSet, I made one in the chart by going to the select data tab and selecting the data bindings button and adding a binding. You can see it there. I then used this value for the second series. Finally, I reversed the values, in the second yaxis, in the chart's script. Hope this helps.