Graphics

Computer Generated x-y Plotting

Using Graphical Analysis for Windows

Hopefully by now you have gained an appreciation for the various elements of an effective scientific graph. These elements include but are not limited to: (a) properly labeled axes, (b) an appropriate title, (c) an effective use of space and proportion in your choice of the scale of the x and y axes, (d) an appropriate line correlating the data points as the case may be.

In modern practice most scientific graphing is done by a computer program. This handout describes the use of a standard graphics program called Graphic Analysis for Windows, by Vernier Software. While this is not the most powerful program for this purpose, it is simple to use and will effectively illustrate the power and simplicity of computer-generated plotting.

After acquiring data in the laboratory for whatever experiment, you will proceed to the ChemTech PC lab (Foster Hall 326) and plot the experimental data. The following is the procedure for that:

1. Log into your university PC Lab Network account, and launch the program by clicking on the START button in the lower left screen, then PROGRAMS, then CHEMISTRY, then Graphic Analysis for Windows.

2. In the upper left are two blank columns of data headed by X and Y. Enter your laboratory data into these columns, putting the independent variable data in the X column and dependent variable data in the Y column. As you do this you will observe the plot immediately being constructed in the window. (For example, if you are plotting Absorbance versus concentration, then Absorbance goes on the y-axis and concentration on the x-axis.)

3. Double-click on the X heading and (a) type in a more appropriate label for the x-axis in the New Name text field, (b) put in appropriate units in the Units text field. Do the same for the Y heading.

4. Click on the graph title in the plot window and key in a more appropriate title if necessary. Keep it brief. Include your initials at the end of the title for later identification of your hardcopy printout.

The power of computer graphing applications is that they do a number of things for you automatically, including (a) choosing the ranges of your x and y axes, (b) connecting the data points. Sometimes these automated choices are not appropriate for your plot, and may be an actual drawback to using the computer.

5. First, consider whether the automatic choices for the range of the x and y axes is what you really want. In some cases, one realizes that the line should pass through or near x,y = (0,0), so it may be good to be able to have the limits set so that both axes start at 0. Many other cases do not require the zero points to be at the origin.

To be able to make your own choice of plot ranges, double click on an axis you wish to rescale, and choose "More X-Axis Options" or "More X-Axis Options". Then key in new top and/or bottom limits.

6. Now comes the question of whether the line connecting the data points is appropriate for this application. If not, in the Graph pull-down menu, turn Connecting Lines option OFF.

7. If you wish to draw a best-fit straight line through the data points, this can be accomplished by "dragging" across the region of the graph you want to fit and using the Analyze pull-down menu to choose Regression. A box appears on the graph showing the slope M of the best fit straight line and the value of the y-intercept B.

8. Sometimes one uses this graph as a calibration plot to interpolate the value of some unknown. For example, in lab you measured the absorbance of the unknown solution. Read the concentration corresponding to that absorbance off your graph by turning on the Interpolate function in the Analyze pull-down menu, and moving the cursor along the best-fit line until you reach a Y-value equal to the absorbance of your unknown. The X-Y position of the cursor will appear below the box on the graph holding your regression data. With your cursor on the correct Y (absorbance), read off the X value (concentration) corresponding to this absorbance.

9. To generate a printout of your graph click on the Print tool button. Be sure to log off your workstation.

Some x-y graphing tips from Dr. Crouse’s web site.

HINTS FROM "THE BEAR" (Chem Alum now professor at Arkansas State - Gary Emmert)

(or how to make graphs that won’t make Dr. Crouse grumpy.)

Title the graph.

Label the axes, including units.

DON’T play "connect-the-dots." Draw the best smooth curve which fits the data. (Not all graphs are straight lines.)

If you use a graphing program, DO NOT let it draw the line, since it will likely do the above.

If you graph by hand, use a paper with appropriately fine grid. DO NOT MAKE YOUR OWN ON BLANK PAPER (I know it’s hard to believe but students have done this in the past!).

Adjust your axes to use the full page, but leave appropriate margins. (Note : that white border on most graph papers should NOT be your margin.)

You may combine multiple graphs on the one page, if they use the same axes. Use different colors of ink and provide a legend.

Last but not least; Like it or not....NEATNESS COUNTS!!!