I'm trying to display an image (chart) that lives outside the public
directory in RAILS_ROOT/private.
The chart contains confidential information and so can't live in /public
or any sub-directories of /public.
The chart needs to be displayed from a partial. The first display of
the page containing the partial is an html render. Subsequently, the
user can select a different chart causing an ajax call which replaces
the content of the image-containing dom element in the partial with a
new chart image.