Sorry for the delay – I have not been able to look into this yet. Could you possibly attach a sample of your data set and a screenshot of the configuration for the chart so I’m 100% sure I can replicate it exactly as you have it set up?
I have now done some testing and it seems there is a bug in the Google Chart API when using “isStacked:percent” as the null values applied by the “Treat zero values as null” setting will be displayed as text and not a proper “null object”. Using “isStacked:true” works as expected.
It’s not much I can do other than hope Google will fix this in a future revision.