Excel normally skips over any missing points (blank cells) in your data range. You have two other options:
Plot the missing points as zeros.
Interpolate the missing points.
If you interpolate, Excel draws a straight line to bridge the gap created by a missing value; it does not actually interpolate a new point. Hence, this option is meaningful only in line, xy (scatter), and radar charts, and only in data series that connect points with lines (that is, it's not meaningful in a series that uses unconnected markers).
To change the way Excel handles missing points in a chart, select the chart, choose Tools, Options, go to the Chart tab (see Figure 24-7), and choose an option button under Plot Empty Cells As.
Figure 24-7. On the ...