[R] Help with wireframe graphics problem (newbie)

[David Wiley]

All,

I've read several tutorials re: generating wireframes, but am clearly
missing something. I have data along the lines of:

> tbl [1:10,]
   Visits Activity Course.Grade
1      17        2        18.31
2       7       11        20.67
3       9       17        24.69
4      28       71        38.72
5      43      107        45.46
6      14        5        47.77
7      25       51        57.81
8      32       27        59.46
9      60       43        64.39
10     31       46        66.26

When I run the command:

wireframe(Course.Grade ~ Visits * Activity, data = tbl)

I get an appropriately labeled, but empty, cube. Can anyone tell me
why there's no data in my cube?

Thanks in advance,

David