This is a feature in many IDEs such as Eclipse and Visual Studio, which I believe can be quite useful in a general purpose text editor.
Along the gutter (next to the line numbers) a colored line is displayed across the lines which have been modified (and unsaved). Often there are multiple levels of this that show different "levels" of modification (unsaved changes, saved but not committed to VCS).
This is very easy to implement in a basic way by using features already present in the editor (indicators), so this can be done as a plugin also. But it's not quite ideal because we don't want to use a circle (the circle would also conflict with other plugins I have like sublimeclang) we want a space-saving 1 or 2 pixel colored line for this.
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