Are the numbers on the side of the chart axis labels that display the day numbers, so that you just want to insert the text "Day " before them? That is easy to do with number formatting. Just do the following:
1. In the chart, right-click on the axis labels and select "Format Axis".
2. Depending on your version of Excel, the next screen will look different. You want to look for the tab or screen option called "Number".
3. In the "Category" box (which lists options like "General", "Number", "Currency", etc.), select "Custom", then click in the input box (either below the category box or to the right of it) where you can type in your own custom format.
4. Enter the following into the box: "Day "0
5. If you have Excel 2007, you also have to click the "Add" button. If you have earlier versions, I think you can skip this step.
6. Click "OK" or "Close"
You should now see your axis displaying the text "Day" in front of the day number. Note that the custom format above does not include the comma between the thousands places. If you have more than 999 days, you may want to add in the comma (to display 1,000 instead of 1000), which you do by entering the following in step 4 instead: "Day "#,##0
You can do similar custom formats to cells - just right click on the cell, select format, and go to the Number tab. You can pretty much put in whatever text you want, either before or after the number. Search the Excel help for "custom number format" for more information.
Another option, if the axis labels aren't the day numbers, is to replace the axis labels in the chart with a set of labels that say exactly what you want. If you don't mind changing the actual data table that you currently have, you can simply replace the numbers with the "Day #" labels that you want. However, if you don't want to mess with the data that you currently have, you can insert labels in another (possibly hidden) column and reference that column in the chart settings. You just have to make sure that the labels are in the same order as the corresponding rows of data. These instructions are different depending on your version of Excel:
For Excel 2007, right-click on the chart, choose "Select Data", and click on the "Edit" button under the "Horizontal (Category) Axis Labels" section. Delete the cell reference that is there and use the mouse to select the column that contains your new labels. Click "OK" twice.
For earlier versions of Excel, right-click on the chart and choose "Source Data". On the screen that comes up, look for the "Category (X) axis labels" box and delete the cell reference that is there. Use the mouse to select the column that contains your new labels, then click "OK" once.
With either method, you should now have the correct labels on your chart.
I hope this helps!