Using LineFit

Figure 1. LineFit opening default window (Mac version)

You may replace the default data with your data by selecting Data from the menu bar, then Edit Datapoints. This will open a new window in which you can enter your data, see Fig. 2.  The number of sets of datapoints must be entered and must not exceed 20. Data may be positive or negative and may be in scientific notation.  For example, -0.00365 may be entered as -3.65e-03.  The e-03 stands for × 10^-03.  DeltaX (Δx) and DeltaY (Δy) are the absolute uncertainties in the data point and must always be non-negative.  The uncertainties may be set to zero if they are not known.   When you are finished entering your data, click on the OK button and you are returned to the graph.
 

Figure 2. Data entry window

When you return to the graph, your data may not be visible, may be partially shown, or scrunched up into one corner.  This occurs because you also need to tell LineFit the range of data to display on the x and y axes.

You indicate the range of values to show on the x and y axes by selecting Data from the menu bar and then Edit Graph.  This brings up the window shown in Fig. 3.
 

Figure 3. Window for editing range of data displayed, the number of tick marks on the axes, and labels.

Choose a range so that all your data is visible and that the datapoints cover most of the graph.  The minimum and maximum values may be positive or negative or in scientific notation. The origin (0, 0) does NOT always have to be visible.   Be careful though.  Sometimes the datapoints cover too much of the graph, as shown in Fig. 4, and the slider cannot be moved far enough to fit a line to the data.  Simply give another range for your data.
 

Figure 4. Poor range selection, slope of best fit line cannot be properly adjusted

The # of X Divisions and the # of Y Divisions boxes in Fig. 3 lets you selects how many tick marks are shown on each axis and where they are placed. The numbers entered here must be positive integers.  You are allowed a maximum of 20 tick marks.  To decide on how many tick marks you need, decide on where you want the main divisions to be. For example, imagine you data that runs from 0 to 50 on the x axis, and that you want tick marks at 10, 20, 30, and 40. The spacing of the tick marks is every 10.  The number of tick marks you want is
 

To finish, you need to an appropriate title and labels on your x and y axes.

There is one last item under Data on the menu bar.  This is Change Mode.  This opens the window shown in Fig. 5.  In the Mac version only the first two choices are working.  You have the option of fitting either one or three lines to the data.  Your lab instructor will indicate which you should select.
 

Figure 5. Choose the number of lines that you can fit to the data

After you fit your line to the data, you will want to print out your graph or compare it to a computer generated best-fit line that uses the Method of Least Squares. In either case, you will first need to save your line.  Select Graph from the menu bar and then Save Line.  Your line will change colour and can no longer be moved around. Similarly, a computer generated best-fit line can be found from Graph and then Best Fit Line.

The File command on the menu bar has a number of important selections. First your may Save your data to a file.  The data files are simple text files but have the .GRF extension.  You will need a 3½ disk if you wish to save your data. You may rename your data with the Save As command. If you have already saved your data to your floppy disk, you may also Open that file again and continue editing.  Your best fit line is not saved with your data however.

Also under the File command are your printing options.  Your may either print a table of your data using Print Data or print out your graph using Print Graph.  Before you print out your data or graph, you should select Page Setup. This will bring up a window, see Fig. 6, that allows you to add a message to the printout, preferably your name, so that you can distinguish your printout from your classmates.
 

Figure 6.  Add your name to your printout with this window.

Finally, Print Setup is the ordinary printer command that allows you to select whether the printout is in portrait or landscape and how many copies to make.