Plotting Time Series on the TI-83/84

Summary:  You can use your TI-83/84 to plot a time series. Enter the dates or times in one list and the values in the other, then make an x-y line plot.

See also:  TI-83/84 Troubleshooting

Let’s plot the closing prices of Cisco Systems stock over a two-year period. The following table is adapted from Sullivan, Michael, Fundamentals of Statistics (Pearson Prentice Hall, 2008), page 82, which credits NASDAQ as the source.

Step 1: Enter dates or times in L1 and values in L2

The first problem is the calculator doesn’t understand dates or times, so you have to change the dates to plain numbers.

The obvious way — 303, 403, and so forth — will lead to wrong results. Look what happens at the end of 2003: you have 1203 followed by 104. Which of those is larger? The calculator will plot the 1/04 closing right next to 1/03 — it will put January 2004 between January and February 2003.

To get around this, you must replace the dates or times with a plain number sequence that keeps the same order. For instance, the months could be numbered 1 to 24, like this:

Now you’re all set. Press [STAT] [ENTER]. Put the month numbers in L1 and the closing prices in L2.

By the way, this note will use L1 and L2, but you can use any lists you like, as long as you enter the actual list numbers in the right places on the Stat Plot screen. (It doesn’t matter whether there are numbers in any other list.)

Step 2: Program the x-y plot on the Stat Plot screen

Turn off other plots.	Press [Y=]. Cursor to each highlighted = sign or Plot number and press [ENTER] to deactivate.
Turn on Stat Plot 1 as an x-y line.	Press [2nd Y= makes STAT PLOT] [1] [ENTER] to turn on plot 1.
Select the “xyline” icon.	[▼] [►] [ENTER]
Answer Xlist: with L1 because the x’s (minth numbers) are in L1.	Press [2nd 1 makes L1] [ENTER].
Answer Ylist: with L2 because the y’s (closing prices) are in L2.	Press [2nd 2 makes L2] [ENTER].
Select a square or cross for Mark, because a dot would be lost within the plotted line.	Press [ENTER].

Step 3: Set up the grid

The grid settings are always a judgment call. For our particular data it seems like a good idea to have a dot every quarter (every three months), so we set Xscl to 3. The prices vary between $12 and $25, give or take, so let’s put a dot every $5 by setting Yscl to 5.   Set only Xscl and Yscl. The values of the other parameters don’t matter (except Xres, which is always 1).   There’s no one right choice for the grid spacing — you need to set it to what seems to make sense in each problem.

Press [WINDOW]. Ignore the min and max lines and set the two scl lines.

Step 4: Display the graph

The calculator can frame the window for you. Use the ZoomStat command.

Press [ZOOM] [9].

If you like, you can trace the graph by pressing [TRACE]. Press [◄] and [►] to move through the time series. To suppress the tracing information, press [GRAPH] again.