Bar graphs are super useful at looking on your data. Often, a 100% bar graph is really needed to compare categories.
In this example, I'm comparing the effectiveness of different messages. Currently, as each message type was sent a different number of times, a precise comparison is not fully visible. If the graph was a 100% graph, the comparison would be obvious and easy. This is why Google Looker studio and other products are offering 100% bar graph and not just the regular bar graph.
Attached is the bar graph from my example, and a (random) 100% bar graph.
