Question about CAS and Graphing

[jebem]

The Home environment requires all variables to be declared before they can be used (CAS is another environment where you can reference the variables without declaring them in first place).
It happens that Home mode has a number of system global variables predefined (the ones in uppercase that you mention like "X").
Try to declare the lowercase variables in Home mode in first place, by storing 0 in them and see if this works for you.