Creating Graphics in Minitab
There are many different applications that create graphics (SAS/GRAPH, Microsoft Excel, Splus and Minitab) but often the most convenient option for our purposes is Minitab Version 11 (soon to be Version 12). We will learn how to do a simple graph, manage graphs on a page and paste graphs into Word documents. Graph types are so different that this will be a far from comprehensive overview but it should give you a good idea of what Minitab can do.
We will use the ``bears" data set in Minitab for our first plot. From the File menu, choose Open Worksheet. In the dialog box, select the Data folder and then select bears.mtw from the Data folder. Take a look at the data-it contains capture data on bears from Pennsylvania. The first plot will be a simple scatter plot with a smoothed curve overlaid. Choose Plot under Graphics and then double-click on Chest.G as the x-axis variable and Weight as the y-axis variable and click OK. Note that the plot could be improved in appearance. You can change the x-axis label by choosing Plot under the Graphics window again; note that the previous settings have been saved. From the Frame menu, choose Axis. Under the Label column, highlight the entry in the first row (Corresponding to the x-variable)-it will read Auto-and enter ``Chest Girth". Minitab has much more flexible column labeling than in the past and relabeling isn't as necessary as before but you'll find you use it frequently. Note all the other options for the axis label-font selections, line styles, axis positioning, etc.
Besides relabeling the axes, it's often handy to fit a line to the curve. We can use LOWESS (LOcally WEighted Scatterplot Smoothing) to fit a nonparametric robust curve estimate to the data. From the Plot dialog box once again, click in the cell for Item 2 and Display. Select Lowess and then click in the adjacent box under For Each and select Graph. While in this row, click the Edit Attributes... button. You can control the degree of smoothing, among other parameters (replacing 0.5 with 0.1 would give you an undersmoothed curve and replacing 0.5 with 0.8 would give you an oversmoothed curve). 0.5 is a reasonable default though I often use a little less smoothing. Plot the data with the Lowess curve and decide whether you're satisfied with the amount of smoothing (i.e., does the curve doa reasonable job predicting weight at all values of chest girth?). Adjust the amount of smoothing if necessary. Each Display item has different attributes to be edited. For example, you can adjust the size and type of symbol plotted if you clicked in row 1 under Item.
Finally, most graphs need a title. Under Annotation, select Title. You can enter multiple titles and then use the dialog box below to adjust the size of the titles. Enter ``Scatterplot and LOWESS curve of Bear Length and Bear Chest Girth" in Row 1 and then ``Pennsylvania Bear Data" in Row 2. Replace the Text Size of 1.5 with 1.0 for Row 2 and then plot the graph.
Multiple Graphs
We can print multiple graphs on the same page using the Layout option in Minitab. You should only use this option after you're satisfied that every graph you plan to include on the page is in reasonable shape since the layout mode only prints your final product (the set of all graphs you've chosen to print). From the Graph menu, choose Layout and then click OK in the dialog box. Choose Plot under the Graph menu once again. Under Regions, select Figure and change Y maximum to 0.50; this plot will occupy the bottom half of the page. Click OK and note that no new graph is produced; layout is ready for your next graph. Plot Length against Weight in much the same fasion as you plotted Chest Grith against Weight. This time set Y maximum to 1.0 and Y minimum to 0.5. Click OK and then select End Layout under the Grpah menu. Your plot will appear on screen. You can use layout to produce other combinations of graphs and they don't have to be the same graph type (i.e., we could mix histograms and scatterplots).
Saving Graphs
Graphs can be saved as Minitab graph files by choosing Save Window As under the File menu and then saving it as a file with a .MGF extension. These file can be opened again Minitab for limited further editing. At this time, they can be pasted into Word by selecting Copy Graph under the Edit menu in Minitab. Switch over to Word and select Paste under the Edit menu. The graph may take awhile to appear-be patient. Selecting Paste Special may give you more attractive text.