Show chart data for empty cells

For example, there are some gaps or empty cells in the chart data table:

If you want to see average values in the chart, do the following:

   1.   Select the chart data by doing one of the following:

• Under Chart Tools, on the Design tab, in the Data group, choose Select Data:
  
• Right-click in the chart area and choose Select Data... in the popup menu:
  

   2.   On the Select Data Source dialog box, click the Hidden and Empty Cells button:

   3.   On the Hidden and Empty Cell Settings dialog box, in the Show empty cell as radio button groups, select Connect data points with line:

Excel will "show" all empty data on your chart:

See also this tip in French: Afficher les données du graphique pour les cellules vides.