Gnuplot maintains a number of read-only variables that reflect the current internal state of the program and the most recent plot. These variables begin with the prefix "GPVAL_". Examples include GPVAL_TERM, GPVAL_X_MIN, GPVAL_X_MAX, GPVAL_Y_MIN. Type `show variables all` to display the complete list and current values. Values related to axes parameters (ranges, log base) are values used during the last plot, not those currently `set`.
The read-only variable GPVAL_ERRNO is set to a non-zero value if any gnuplot command terminates early due to an error. The most recent error message is stored in the string variable GPVAL_ERRMSG. Both GPVAL_ERRNO and GPVAL_ERRMSG can be cleared using the command `reset errors`.
Interactive terminals with `mouse` functionality maintain read-only variables with the prefix "MOUSE_". See variables for details.
The fit mechanism uses several variables with names that begin "FIT_". It is safest to avoid using such names. "FIT_LIMIT", however, is one that you may wish to redefine. Under `set fit errorvariables`, the error for each fitted parameter will be stored in a variable named like the parameter, but with "_err" appended. See the documentation on fit for details.