Make loading of gnuplot work with and without gnuplot-context.
This is a bit tricky and there may well be a better solution. Here's how it works: - gnuplot-context.el is not loaded automatically by gnuplot.el - function `gnuplot-context-sensitive-mode', the main on/off switch for contextual parsing, is defined in `gnuplot-context.el' and marked as autoloaded in `gnuplot.el'. This makes the menu item and M-x gnuplot-context-sensitive-mode work, by loading the gnuplot-context library as soon as it's needed - defcustoms relating to context-sensitive mode are in gnuplot.el, so that context mode can be discovered and enabled without an explicit (require 'gnuplot-context). A special :set function for the defcustom `gnuplot-context-sensitive-mode' loads gnuplot-context.el only if enabling the mode. defcustom has a :require option which seems like it would be made for this, but in fact it loads the library independent of whether it's enabled or not, AFAICT.
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