Please refer to Rule 10 §57. I would interpret this to mean that you should use the general omission symbol. If in doubt, you could add a transcriber's note explaining the print use of the question mark.
Please note that I was looking at this from the perspective of an actual tactile graphic of the bar graph. If you are using braille dots (allowed for grade 4 and up) to create the bars, you would not be able to use the general omission symbol as that would result in an inacurate representation of the graph data! edited by betty.marshall on 6/29/2011