Just installed 2.7 b290. Added a new bar graph to a page and could not find the 100% bars button in the toolbar. When I clicked the Cascade button, the 100% button appeared and I could change the graph to 100% (cascaded). I toggled Cascade off and the 100% button disappeared from the toolbar - as did the 100% feature of the graph. I got a regular bar chart.
As an experiment, I created a new page and added a bar chart. Again, the 100% button was missing but this time when I followed the steps outlined above I managed to get 100% bars (not cascaded).
p.s. Just installed b 294 and have the same issue.
100% bars is only available when you have cascading and/or stacking. If you don't have any of these, all the bars would be 100% high which would have no use, hence it is unavailable.
Please post a file for which I can follow your steps precisely. I've tried it with the Bond Prices demo file without success.
Now I can't recreate the problem. I was in a mad rush this morning to get a report finished and I suspect that I may have tried to apply 100% bars before I'd set the stacks up. D'oh!