it is not possible to do it on a Bar chart Trelli...However, i was facing the same challenge last week and i find a way to avoid this problem and to have a total within the bar chart. see the image.
You see the yellow figure it is a trick. i have created a TEXT OBJECT with a formula in it, and i have set the color to transparent. therefore, i can put it on the top of the Bar Chart Trelli.
Hope this helps even if it was a long time ago.