Only channels can be indexed by frame, e.g.:

This does not work:

// variable created during runtime:

my_variable = time * 2 + 100;

a = my_variable[ frame - 1 ];

To add a custom variable that then will be indexable by frame, refer to "adding custom channels" in the documentation.

// "my_variable" is a custom channel of this plugin:

Xpressionist.my_variable = time * 2 + 100;

a = Xpressionist.my_variable[ frame - 1 ];