How do I slap a consistent footer onto an Angular Material table in the name of having, for example, a sales total for all sales?

Below the closing for the table detail which has an opening like so...

<td mat-cell *matCellDef=

 
 

...you want:

<td mat-footer-cell *matFooterCellDef>
   Do what you will...
</td>

 
 

Below this row...

<tr mat-row *matRowDef="let row; columns: displayedColumns"></tr>

 
 

...you want:

<tr mat-footer-row *matFooterRowDef="displayedColumns"></tr>