Bar graphs are easy to understand, widely used, and can show changes over time. That gives them an advantage over other graphs that are difficult to read or can only show a single data set.
They use vertical or horizontal bars to represent data along both an x-axis and a y-axis visually. Each bar represents one value, so it'll be an advantage for Peter because he can add as many colours he wants and the graph would still be easy to read. When the bars are stacked next to one another, the viewer can compare the different bars, or values, at a glance.
