You might be thinking of the scattergram.
a double bar graph
The line and the bar graph is used to describe a graph that compares two sets of data.
The difference between a bar graph and a double bar graph is...... A bar graph displays one set of data, and a double bar graph compares two different sets of information/data.So should have a key.
bar graph
double line graph
You can compare two different data sets on the same chart.
You can compare two different data sets on the same chart.
The advantage is that you can compare multiple data sets in the same chart
line graph
a bar graph
Suppose you plotted the same pairs of variables from two data sets on the same scattter plot, perhaps representing the two data sets in distinct colours. Then if the data sets differed sufficiently in standard deviation you would notice greater scatter or spread in one of them.