Hi @phil.miller, thanks for posting in the Developer community! 
 
If you are trying to SUM values in individual cells of a table in Lucidchart, you can achieve this by adding some custom data to the table to represent the value in each cell.
 
You can reference individuals cells in a table using A1, A2, A3, B1, B2, B3 etc. In the custom shape data for your table, you can create a property field for each cell you want to include in the SUM. As the value for that field, I added a formula to reference the number on that particular cell: =this!A2 for cell A2, =this!B2 for cell B2 and so on. 

Then I added this formula to the cell I wanted to display the count in:  =SUM(@"A2",@"B2",@"C2").

 
I hope this helps! If this doesn’t address what you are looking to achieve or you have any questions, feel free to comment below.