Included with emacs is a basic tool called
To edit a picture made out of text characters (for example, a picture of
the division of a register into fields, as a comment in a program), use
the command ‘M-x picture-mode’ to enter Picture mode.
In Picture mode, editing is based on the “quarter-plane” model of
text, according to which the text characters lie studded on an area that
stretches infinitely far to the right and downward. The concept of the
end of a line does not exist in this model; the most you can say is
where the last nonblank character on the line is found.
See the manual for more information.
The manual also mentions the existence of file
artist.el that provides more sophisticated operations through
artist-mode. Here's an extract of the docstring:
Artist lets you draw lines, squares, rectangles and poly-lines,
ellipses and circles with your mouse and/or keyboard.
Drawing with the mouse:
Pops up a menu where you can select what to draw with
mouse-1, and where you can do some settings (described
Draws lines, rectangles or poly-lines, erases, cuts, copies