Graphing in Excel (Scatter Plot)

Transcription

1 Graphing in Excel (Scatter Plot) Introduction This is a short, step-by-step tutorial on how to make a scatter plot in Excel. Readers are expected to know basic graphing skills but no Excel experience is required. Procedure 1. Open an Excel spreadsheet. 2. Choose a place to begin and enter the data values for your x-axis in one column and then enter the corresponding y-axis data values in the adjacent column. You can enter your values anywhere on the spreadsheet but aligning the columns will help avoid errors. It s good practice to label rows and/or columns in the spreadsheet for clarity. Refer to Figure 1 on the right for an example. Note: Each x value must have a corresponding y value and vice versa for a continuous plot. 3. Click the Chart icon from the Toolbar menu. Alternatively: Select the Insert pull-down menu, and then select Chart. Figure 1 Spreadsheet 4. A window similar to the window displayed in Figure 2 (to the left) should pop up. Under the Standard Types tab select the chart type and the chart subtype. 5. Click next to proceed to the Chart Source Data window then click the Series tab in the upper left-hand corner of the window. 6. Click the Add button in the middle-left of the window. Figure 2 Chart Type The window should now appear similar to the window in Figure 3 on the next page.

2 A few comments about Series in Excel: Each column of y-axis data is called a series. If you are plotting multiple curves, you can select different series in the Series selection box in the middle left of the window. There are 3 places to input data to the right of the Series selection box. The Names and Y Values inputs are associated with the highlighted series in the Series selection box. X Values will need to be entered for every series when doing a scatter plot. However, it should be noted that for a line plot the data in the X Values input data box will be the same for all series plotted. Typically, these 3 data input boxes contain spreadsheet equations or addresses to locations on the spreadsheet that contain the numeric data for the Figure 3 Chart Source Data plot. Instead of working with these equations, Excel provides an icon to the right of each input data box that allows the user to access the spreadsheet and highlight data points. However, you can directly type the name of the series in the Name input box and it will have the same effect as selecting a cell on the spreadsheet with the series name typed in it. 7. Select the icon to the right of the X Values data input box. The window will reduce to a small data input box, you will now be able to select cells on the spreadsheet and view the equation that references those cells in the data input box. 8. Highlight the column/row of x values, then close the small data input box by clicking on the icon on the right. 9. Select the icon to the right of the Y Values data input box, highlight a column of y values, and click the icon to the right in the reduced window (same as step 8). 10. When plotting multiple series it s usually necessary to name each series. If only one series is to be plotted, skip to step 12. There are two ways to enter a series name: (a) type the name directly into the Names input data box or (b) select the icon to the right of the Names data input box and select a cell in the spreadsheet that contains the series name. 11. Click the Add button to add another series and repeat steps 7 thru 10 for each series added. 12. Click Next; this will bring you to a window called Chart Options. 13. Click the Titles tab and enter the title of the graph in the Chart title input data box. Enter the x and y axis labels in the Values (X) axis and Values (Y) axis data input boxes. Axis and gridlines properties can be changed under the Axis and Gridlines tabs in this window. 14. If more than one series is plotted it may be necessary to have a key or legend. Under the Legend tab the user can choose where on the graph the legend should appear. If

3 only one series is to be plotted the user can choose not to show the legend by unchecking the Show legend box. 15. The Data Labels and Data Tables tab allow the user to add more clarity to their graphs if needed. Under the Data Labels tab the user can choose to display x or y data next to each data point. The Data Tables tab (not an option if plotting a scatter plot) the user can insert a table of the numeric data into the graph. 16. Click next 17. Select the bullet labeled As new sheet, then click Finish. The graph should now appear in its own sheet. The user still has the ability to return to any of the windows used to create the plot by right clicking somewhere on the graph (avoid gridlines and axes) and selecting an option from the list.

4 Cautions and Tips 1. To remove gray background (saves ink when printing) -Right click somewhere on the graph, avoiding gridlines and axes -Click Clear 2. To format title and axis titles -Right click on the title -Select Format Axis Title or Format Chart Title -Change font, alignment, color, etc. as needed -If titles are not present on graph -Right click of graph, avoiding gridlines and axes -Select Chart Options -Under Titles tab enter x and y axis titles and a title for the graph 3. To format data series -Right click on data point of the data series to be formatted -Click Format Data Series -Change patterns and colors, series order (as displayed in legend), etc. 4. Axis scale caution -Be conscious of axis scale Excel auto-scales both axes to fit the range of data, this may lead to misconceptions of what actually is plotted -Example -Data taken from an experiment was entered into and excel spreadsheet (Figure 4) and plotted (Figure 5). Both graphs plot identical information and trend lines have been added, the graph on the left has been auto-scaled by excel and the graph on the right has been modified by the user. Figure 4 Scale caution data example -The point of this example is to illustrate how data can appear to be something it is not. In this example data was plotted from an arbitrary experiment which resulted in a linear line with zero slope. However, the data that was auto-scaled appears random, yet it is identical to the graph on the right. Figure 5 Scale caution graphs

5 5. To add a trendline (Regression fit) -Right click on a data point -Click Add Trendline -Select trendline that best describes data set -To display equation on graph -Click Options tab -Check Display equation on chart -Click OK 6. To linearize data set -With x and y data entered into the spreadsheet, click on a cell adjacent to x data (Figure 5.a). -Type: =Ax*Ax, where A is the column letter and x is the row number (Figure 5.b). -Click and hold the small black square in the bottom right corner of cell, drag down entire length of column and release (Figure 5.c). -Return to step 3 of procedure to plot data a) b) c) Figure 5 Linearization