1. The Basic X-Y Scatter Plot

Transcription

1 1. The Basic X-Y Scatter Plot EXCEL offers a wide range of plots; however, this discussion will be restricted to generating XY scatter plots in various formats. The easiest way to begin is to highlight the columns of data you want to plot. This can be done by highlighting the first column and then, while pressing the Cntl key, highlighting the second column. This can be repeated for as many columns as you want. We will start with just two columns to keep the process simple. The highlighted columns are illustrated in Figure 2.1. With the desired columns highlighted, select the Chart Wizard from the tool bar. See the large arrow in Figure 2.1. Figure 2.1 Selecting data for plotting ExcelGraphs.doc, 4/2/04, 1:51 PM 1 of 16

2 From the Chart Wizard, select the XY (Scatter) option as illustrated in Figure 2.2. Click next, select the Series Tab, and you should see something like the dialog box in Figure 2.3. Figure 2.2 Chart Wizard ExcelGraphs.doc, 4/2/04, 1:51 PM 2 of 16

3 Notice how Name, X-values, and Y-values are filled in. If for some reason, your vertical axis is in the X-values and your horizontal axis is in the Y-values, you can change them at this point. In the present case, they are the way we want them with altitude on the vertical axis. From this point forward, the process is one of refining the graph by adding more data and cleaning up the presentation. Let s begin by adding the dew point temperature, column DWPC. Figure 2.3 Step 2 of 4 in the Chart Wizard ExcelGraphs.doc, 4/2/04, 1:51 PM 3 of 16

4 In the dialog box illustrated in Figure 2.4, under Series select Add. We want to add a new series for dew point. Click in the name field to activate it, then click on the name in the spreadsheet. Similarly, click on the X Value field then highlight all the dew point temperatures. Repeat the process for the HGHT which goes in the Y values. The dialog box should appear as in Figure 2.4. Our graph is starting to take shape, but we will now click next to continue refining it. Keep in mind that you could have started by selecting all three columns to begin with, but it is often easier to start with just two and add from there. Figure 2.4 Adding Another Data Series ExcelGraphs.doc, 4/2/04, 1:51 PM 4 of 16

5 After clicking Next, you should see Step 3 of the Wizard. At this point, we will do a little tweaking, but most of the clean up will be done after we actually generate the graph. For starters, click on Titles and enter some labels as illustrated in Figure 2.5. We will now click Next and choose the option to create a new object in our existing worksheet. We could alternatively put the graph in a separate worksheet, but I think we have more options if we keep the graph as an object in our current worksheet. Try both and see which way you like. You could stop here, but the graph is not very attractive. Let s continue by polishing the appearance of the graph. Figure 2.5 Step 3, Chart Options ExcelGraphs.doc, 4/2/04, 1:51 PM 5 of 16

6 2. Polishing the Graph At this point in Figure 3.1, the shape of the data is evident, but the appearance is not very attractive. We will now polish the appearance. This process can be very tedious and time consuming so everyone will choose a different stopping point. First, notice that when you click on the chart in your worksheet, the Data menu item is replaced with a Chart menu item. Click on Chart and take a look at it. From here you can change anything you have already done. Two options are of particular interest. Source Data Chart Options Source Data activates the dialog box shown in Figures 2.4 and allows you to change or add to the series. Chart Options activates the dialog box shown in Figure 2.5 and allows you to change titles, legends, etc. We are going to polish our graph using a new set of dialogs which are activated by clicking directly on the graph. Radiosonde Data 2/16/04 IAD Altitude Meters Temperature Celsius HGHT DWPC Figure 3.1 The first version of our graph ExcelGraphs.doc, 4/2/04, 1:51 PM 6 of 16

7 By double clicking on the graph itself, we can change the appearance of any item in the graph. The following dialog box and options are available. Dialog Box & Options Format Data Series Format Axis Format Plot Area Format Chart Area Format Chart Title Format Axis Title Format Legend Activate by Clicking on One of the chart data points Either the X or the Y axis An empty space in the plot An empty space outside the plot Either the X or the Y title The plot title The legend We will apply each one of these to polish the appearance of our graph. Let s start with the legend for the Y-axis. 2.1 Format Axis The first thing we will do is format the Y-axis legend. We will do this by double-clicking on the Y-axis legend and activating the Format Axis dialog box as shown in Figure 3.2. We changed the major scale increment from 5000 to 2000 and said that we wanted to display the data in units of thousands. Under Number, we changed the decimal places from two to zero. Under Patterns, we changed the tick marks to low to move them over to the left hand side of the graph. We then clicked on the X-axis and changed its major units to 10. Figure 3.2 Format Axis ExcelGraphs.doc, 4/2/04, 1:51 PM 7 of 16

8 The graph is now starting to look a little more presentable as illustrated in Figure 3.3. We could certainly stop at this point but let s continue with a few more refinements. Altitude 1000's Meters Radiosonde Data 2/16/04 IAD Temperature Celsius HGHT DWPC Figure 3.3 Version 2 of Graph ExcelGraphs.doc, 4/2/04, 1:51 PM 8 of 16

9 2.2 Format Data Series Let s change the symbols used to plot the points. We would like to have the temperatures plotted as X s and the dew points plotted as triangles. We can do this by double-clicking on one of the data point markers and activating the Format Data Series dialog box shown in Figure 3.4. The marker symbols can be changed in the Patterns tab. Note, if you wanted to connect the points with a line, you would also do that here. Figure 3.4 Format Data Series ExcelGraphs.doc, 4/2/04, 1:51 PM 9 of 16

10 Our graph now appears as in Figure 3.5. We will continue to jazz up the colors with the Format Chart Area and Format Plot Area and use the drawing tool to add some labels. See Figure 3.6. Altitude 1000's Meters Radiosonde Data 2/16/04 IAD Temperature Celsius TMPC DWPC Figure 3.5 Version 3 of Graph Altitude 1000's Meters Radiosonde Data 2/16/04 IAD Troposphere Tropopause Temperature Celsius Stratoshpere TMPC DWPC Figure 4.6 Version 4 of Graph ExcelGraphs.doc, 4/2/04, 1:51 PM 10 of 16

11 3. Adding Data to the Graph 3.1 Adding a New Variable with a Different Number of Data Points Now let s add some data to our graph which has a different number of data points and but has basically the same range as the original data. EXCEL is extremely flexible in that it allows us to add a completely different series. In the case at hand, we are going to add what is called standard pressure data. It has been added to the EXCEL spread sheet manually. Notice that these data points correspond to an idealized set of altitudes and do not have to match one-for-one with the original data. To add the data, we click on the chart, select the Chart menu option, and then select Source Data. The dialog box is illustrated in Figure 4.1. Notice that while our original data went to row 51, this data only goes to row 32. EXCEL takes car of everything and generates the graph along with our existing graph. Figure 4.1 Adding a new series with a different number of data points ExcelGraphs.doc, 4/2/04, 1:51 PM 11 of 16

12 The resulting graph is illustrated in Figure 4.2. We call this Version 5 of our graph. Notice that in this graph, we did not use any markers for the data points and just used a line to show the relationship. This kept the graph much neater less cluttered. Radiosonde Data 2/16/04 IAD Altitude 1000's Meters Troposphere Stratoshpere Tropopause Temperature Celsius TMPC DWPC STTMPC Figure 4.2 Version 5 of Graph ExcelGraphs.doc, 4/2/04, 1:51 PM 12 of 16

13 3.2 Adding a New Variable with a Different X-Axis This next operation is a much more complicated and messy operation. We are going to add a new variable called standard pressure. It varies by altitude, as does temperature, but it has a completely different range of X-axis values. Its values range from 1000 to 55 millibars as altitude increases to 20,000 meters. This means that we have to add a second X-axis to our graph. We again use the Source Data dialog to add the data. Once the standard pressure data is added we get the awful looking graph in Figure 4.3.The first thing to notice is how smashed together the temperature data points are. This is because, EXCEL is trying to use the same scale on the X-axis to plot both temperature and pressure. We need to assign the pressure to a new axis with its own range of values. We also need to clean up the axis labels. Altitude 1000's Meters Radiosonde Data 2/16/04 IAD Troposphere Stratoshpere Tropopause Temperature Celsius TMPC DWPC STTMPC STPRES Figure 4.3 Version 6 of Graph ExcelGraphs.doc, 4/2/04, 1:51 PM 13 of 16

14 Two steps are required to assign a second axis to the pressure data: Assign pressure to a second axis in Format Data Series Create a second axis in Chart Options Click on the data point marks for pressure. When the Format Data Series dialog box appears, select the Axis tab and select Secondary axis. See Figure 4.4. Figure 4.4 Assign variable to secondary axis ExcelGraphs.doc, 4/2/04, 1:51 PM 14 of 16

15 Now click on the Chart menu item and select Chart Options. Select the Axes tab and check Secondary axis. This should pretty much do the job. There may be some minor touch up required to change the scales, but it is just a variation of all the things we have already done. Before adding a second axis you might want to ask yourself if the added chart complexity is worth it. Of course it can be a powerful visual add to see how two different variables vary with a third variable. Figure 4.5 Adding a secondary axis in Chart Options ExcelGraphs.doc, 4/2/04, 1:51 PM 15 of 16

16 3.3 Final Graph The final graph is illustrated below in Figure 4.6. Radiosonde Data 2/16/04 IAD Altitude 1000's Meters Troposphere Tropopause Stratoshpere Temperature Celsius TMPC DWPC STTMPC STPRES Figure 4.6 Version 7 of Graph ExcelGraphs.doc, 4/2/04, 1:51 PM 16 of 16