A line graph is a graph that charts the relationship between two variables or a progression of a single quantity through time. Without the lines connecting points of data, it would be difficult to make sense of the data being presented if there was no overall trend between the dots. A bar graph, on the other hand, is meant to compare two or more statistics, such as population figures.
Line graphs are used in science and statistics to help make sense of data. For example, a sales department might want a line graph to show the overall increase or decrease in profit from one interval to another. A scientist, meanwhile, might want to study the relationship between a material's electrical conductivity and its temperature. The graph would show heat as the independent variable and conductivity as the dependent variable. By drawing points of data, the scientist could then determine a positive or negative trend; in this case, there would be a positive trend, meaning the line would go up and to the right. The precursor to a line graph is a scatter plot, which simply shows the points of data on the graph before adding a regression line to mark the overall trend.