A line plot shows the frequency of data, in short it shows how many times something has occurred or the number of times data occurs in a group of data.
The number of data is represented by markers or in this case "x"s. The more x's along the column, the more times it occurred.
The bar graph there does not show the frequency of the different scores. This shows the relationship between the test number and the percentage of the scores obtained. It does not tell you the individual scores obtained.
