How can you use a bar diagram to show a percent of change?

Answer 1

Yes, but not a single bar. You need stacked bars or, preferably, a pair of bars. With stacked bars you would have the original quantity as the lower bar, and the increased quantity (after the percentage change) as the stacked bar on top. This method does not really work if the percentage change is a decrease.
With a pair of bars, the first would show the quantity before the change and the second would show the total quantity after the percentage change. So, if it was a percentage increase, the second bar would be taller, and if it was a percentage decrease, it would be shorter.