As of right now bar charts have the X and Y axis scale set to be relative to the maximum value within the result set. This makes sense for scales measuring counts or values but not percentages. In the case of latter you would generally want the max to be 100% and not the max percent value of the result set.
It would be great if WF would allow the report creator to check a box and set a fixed max value for the X and Y axis scales so instead of this:
you could have this: