10.2+Line+Graphs

Section 10.2 - Line Graphs
Line graphs are built from paired data, and occasionally from tables of data. They are almost always used with numerical data, at least for one of the two pieces in the paired data, and they are often used to represent change over time.

The example below follows both of those conventions, showing the number of days a home spends on the market for the given months. Different lines on this graph show different information, so they are like different columns of table data.



This table includes the total for each month, but most line graphs do not. If you want to know the exact (or almost exact) total along any part of the line, you would be expected to look over at the left axis and estimate the total.

I see a problem with that, however. It says that in September of 06 the average days on market for single family homes was 59. The next month, it was up to 72. Well, what happened between September and October? Obviously there is no time between the two months, but the line goes up from 59 to 72 to suggest a climb in the days on market between the months. Actually the climb is in the average for the months, not necessarily even the middle of the month. So the line isn't really accurate, only the points on the month lines. The lines connecting the points are just for looks to help you see whether the points were going up or down.

Questions that might typically be asked about this kind of graph are:
 * On average, how many more days was a condo on the market in March of 2007 than a single family home?
 * What was the difference in average days on market for condos in July 06 as opposed to April 07?


 * Strengths of Line Graphs: **

Great for showing trends over time. Easy to make.


 * Weaknesses of Line Graphs: **

Can't show too many categories at once or it becomes very confusing, hard to be exact or precise.


 * Chapter Ten Practice Problems**

**Where to from here?**
10.1 Tables 10.2 Line Graphs 10.3 Bar Graphs

10.5 Circle Graphs Chapter Ten Summary Chapter Ten Homework