Specify a file from which user-defined marker data
(locations and names) should be read. Each line in the
marker data file consists of three required components:
the latitude and longitude (expressed in decimal
degrees) followed by the text of the label that should
be used. Individual components are delimited by either
whitespace, forward slashes (/), or commas. Components
that need to include delimiter characters (e.g., a
multi-word label) should be enclosed in double quotes.
For example, a line in a typical marker data file might
look something like:

    42.33 -71.08 "Boston, MA"    # USA

Everything between a `#' character and the end of a
line, inclusive, is a considered to be a comment. Blank
lines and lines containing only comments are allowed.

In addition to the three required components, xearth
supports optional following "key=value" components. In
this version of xearth, the only supported "key" is
"align", which can be used to control where marker
labels are drawn in relation to the marker proper. Sup-
ported alignment values are "left", "right", "above",
and "below"; the default behavior (if no alignment is
specified) is "align=right".

The marker data file is reread every time xearth
redraws an image into the X root window. In this way,
the marker positions and labels can be dynamic (e.g.,
given appropriate data sources, markers could be used
to encode hurricane positions, where earthquakes have
happened recently, temperatures at fixed locations, or
other forms of "real-time" data).

Xearth includes a built-in set of marker data for 76
major locations around the world. The built-in data can
be selected by specifying "built-in" for the file argu-
ment; this is the default behavior.