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Re: Drawing the equator

Hi Mark,

	Extremely simple. So let me take this opportunity to answer your
question and expand on it just a bit.

	The simplest way to draw the equator is to issue the command:

yes? ppl cross,1

This command is robust and will apply to every following plot until you exit
ferret or issue the command:

yes? ppl cross,0

The other way to draw lines is to use the aline command. This is part of the
PPL+ enhancements added by Jerry Davison. The format of the command is:

ALINE/qualifier line#, minx, miny, maxx, maxy, set

This command draws the line associated with the specified line number between 2
points (see PEN for more on this). Two modes are available. In immediate mode
the line is drawn when the command is given. Deferred mode permits setting of
several lines  (with individual endpoints) to be drawn whenever the PLOT
command is given. Deferred mode is included so that examples of each linetype
used in a plot can be provided as part of a key. The ALINE command does not
modify data in the plot buffer; lines drawn can be considered as labels. The
ALINE command given with no arguments resets all set lines to OFF.

 line# The line to be drawn will be of the type, thickness, and color
       associated with this line number.

 minx  X-component of the first endpoint.

 miny  Y-component of the first endpoint.

 maxx  X-component of the second endpoint.

 maxy  Y-component of the second endpoint.

set is optional. If omitted, execution mode is immediate. If ON, sets deferred
mode for the specified line number. If OFF, drawing the line is cancelled;
specification of the endpoints may be omitted when cancelling ALINE for a
line. Execute LIST ALINE to find which lines are set, and their coordinates.

 Valid qualifier:

 /[no]user determines whether user coordinates or inches will be used in
           locating the line.  Default is /user.

This information is in the pplus_enhance_user_guide, a very useful extension
that adds commands to control things like the color key, and the shade/fill
color spectrum.

Hope this helps, Mark

On Apr 16,  1:13pm, Mark Williams wrote:
> Subject: Drawing the equator
> Dear Ferret users
> Can anyone tell me a quick way to draw a line across a plot to show where x
> or y is zero when it's not on the boundary of the plot?  For example the
> equator on a plot which extends from 30s to 30n north or where U = 0 on a
> plot which has the y axis extending from -20 to 20?

 Mark Verschell                     NASA/Goddard Space Flight Center
 verschell@neptune.gsfc.nasa.gov    Code 970/Lab. for Hydrospheric Proc.
 301-614-5669  Fax: 301-614-5666    Greenbelt, MD 20771

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