XY Lines

An XY line is the most effective way to display the relation between two quantitative variables. 

An XY line is a graphical representation of two quantitative variables on the same coordinate system.  If either variable is a measurement of time, this variable should be plotted on the horizontal axis, or x-axis. 

A common application of the XY line is the time series plot.  The time series plot is a scatterplot with the year, or some other tine period, on the horizontal axis and a value for each time on the vertical axis. 

Typically, the time periods are equally spaced, for example one day, or one year, or ten years.  In such plots, connecting the points from one time period to the next often helps show the general trend.  In reviewing the time series plot, look for places where the general trend seems to change, and try to find possible explanations.  If there is more than one time series on a plot, compare them to determine similarities and differences.  Sometimes it helps to make a new plot of the differences or ratios between the time series.  The majority of all time series do not follow a straight line trend over a long period of time, so fitting a straight line to the data would not be useful. 
 

Steps for graphing the XY line: 

1. Enter the data set below into lists L1, L2, and L3 by pressing  and 1:Edit. 
 
 

2. Press , , and  to set up Plot 1.  Press  to turn on Plot 1.  Press ,   and  to select the XY line graph. 

3. Check that the "Xlist" has list L1 selected and the "Ylist" has list L2 selected for Plot 1.  This directs the calculator to create an XY line graph of the Verbal SAT data by year. 

4. Select the box in the mark selection. 

5. Press  to graph the XY line.  The graph should look like the image below.

6. Press , , and  to set up Plot 2.  Press  to turn on Plot 2.  Press , , and  to select the XY line graph. 

7. Check that the "Xlist" has list L1 selected and the "Ylist" has list L3 selected for Plot 1.  This directs the calculator to create an XY line graph of the Mathematics SAT data by year. 

8. Select the cross in the mark selection. 

9. Press  to graph the XY line for the mathematics scores by year.  The graph should look like the image below.  

 below.  

10. With both plots 1 and plot 2 turned on, press  to graph both XY lines on the same screen.  Press  and 9: ZoomStat to view the XY lines in the appropriate window.  The XY line graphs should look like the image shown above. 

The bottom XY line represents the Verbal SAT scores by year and the top XY line represent the Mathematics SAT scores by year.