Here is the gist of how EXPGUI works.

* read the .exp file into exparray (expload)
* map the phases & histograms etc with expmap (mapexp)

Here is the tricky part: 

  * I have a set of routines (----info) that read and write various parameters

  * To "map" values from exparray to tcl variables (so that they get updated in the exparray as one edits them), the variables are placed in 
array entryvar and array entrycmd is used to hold the command to set the value. 
  * A trace on entryvar is used to execute the appropriate entrycmd code any time a variable is set [in entvartrace]

Example:
    set entryvar(cycles) [expinfo cycles]
    set entrycmd(cycles) "expinfo cycles"

    this uses expinfo cycles to get the number of ls cycles
    if entryvar(cycles) is changed (by changing the contents of the
    entry associated with the variable), then in entvartrace the command 
	expinfo cycles set $entryvar(cycles) 
    is executed to put the value into the exparray and flag that a change 
    has been made.

This is why the first step is to write a ----info routine where
	you get the value using    ---info var <otherparms>
	and set it using           ---info var <otherparms> set value

The next step is to map the various parameters (like P.O. axis) to unique
array elements in entryvar & entrycmd

And the last step is to create a GUI with various widgets that allow the user to change the appropriate values. In this way, what you change does into the .exp file as soon as you save it.

In some cases, where there is a toplevel with a "Save Changes" & "Cancel Changes" button (e.g. Background, Constraints, changes are done a bit more straightforwardly.