Once you have collected data you will want to plot a graph or chart to show trends or relationships clearly. With a little effort, Excel produces very nice charts. First enter the data you want to plot into two columns (or rows) and select them.

Drawing the Graph. Click on the chart wizard . This has four steps:

1. Graph Type. For a line or scatter graph (the most common kind) choose XY(Scatter) (not line) and press Next. This is the only graph that plots one set of data against another.
2. Source Data. If the sample graph looks OK, just hit Next. If it looks wrong you can correct it by clicking on the Series tab, then the red arrow in the X Values box, then highlight the cells containing the X data on the spreadsheet. Repeat for the Y Values box.
3. Chart Options. You can do these now or change them later, but you should at least enter suitable titles for the graph and the axes and probably turn off the gridlines and legend.
4. Graph Location. Just hit Finish. This puts the chart beside the data so you can see both.

Changing the Graph. Once you have drawn the graph, you can now change any aspect of it by double-clicking (or sometimes right-clicking) on the part you want to change. For example you can:

• move and re-shape the graph
• change the background colour (white is usually best!)
• change the shape and size of the markers (dots)
• change the axes scales and tick marks
• add a trend line or error bars (see below)

Lines. To draw a straight "line of best fit" right click on a point, select Add Trendline, and choose linear. In the option tab you can force it to go through the origin if you think it should, and you can even have it print the line equation if you are interested in the slope or intercept of the trend line. If instead you want to "join the dots" (and you don't often) double-click on a point and set line to automatic.

Error bars. These are used to show the confidence intervals on the graph. You must already have entered the 95% confidence limits on the spreadsheet beside the X and Y data columns. Then double-click on the points on the graph to get the Format Data Series dialog box and choose the Y Error Bars tab. Click on the red arrow in the Custom + box, and highlight the range of cells containing your confidence limits. Repeat for the Custom - box.