Update chart.el

Update chart.el

* lisp/emacs-lisp/chart.el (chart-mode): Derive from special-mode.
(chart-draw): Wrap in with-silent-modifications.  Instead of inserting a
fixed number of newlines, use window-height.
(chart-bar):
(chart-trim): Use dolist.
(chart-file-count): The previous implementation was buggy and missed
extensions.  Use file-name-extension instead to detect file extensions.
Also use dolist and cl-incf to reduce verbosity.