As you have already discovered, a BYTE can contain 256 values (0 to 255), and an INTEGER consists of 2 BYTES.
So, an INTEGER can contain 256*256=65536 WHOLE NUMBER values.
If you do not assign the variable type Excel automatically assigns the VARIANT type.
I would just say that defining your variables as certain types is just good practice.
Others may want to step in with their own points of view.
Maybe this topic is not the place for this discussion and another could be created.
Back to the charts....
Excel will need to know the starting point, that is, the date that the first column number.
From there a simple formula could be constructed to determine the current month value.
e.g.
Starting Month=Jan 2010
Current Month=April 2011
Current Month-Starting Month=difference in months = 15
Therefore x= Chart column offset =15
To do this in VBA...
DiffMonths = DateDiff("M", startmonth, currentmonth)