"If the datasheet data for the category axis contains date number formatting, Microsoft Graph automatically uses a special type of axis in your chart called a time-scale axis.
A time-scale axis shows a blank category for dates for which you have no data. If you do not want to see these gaps — for example, if you have data for 1-Jan, 15-Jan, 3-Feb, 12-Feb, and 2-Mar, and you want to plot the days next to each other - you can change the time-scale axis to a standard category axis.
On the Chart menu, click Chart Options, click the Axes tab, and then click Category under Category (X) axis."
Note, For 2007+, on the Design tab, click Select Data in the Data area, and then click Hidden and Empty Cells on the Select Data dialog box.