I'm having problems with the axis synchronization in Tableau 10. I think it may be a bug but a second opinion would be appreciated!
When plotting measures on left and right axis of a chart, if the axis are synchronized and the measure on the left axis = null, the measure on the right axis does not plot properly.
Please see workbook attached. In this example I have mimicked the scenario above for 'Customer A'. Notice that when you hover over any of the data points they do not match the axis, unless you switch off synchronization.
Switch to 'Customer B' where the left axis contains values and the chart works fine.
If you convert your NULL to 0 that should fix it, for example instead of having M1 on the axis create a calculated field ZN([M1]) to convert it to 0 and put that field in the place of your M1 field.