How does GrADS work internally when drawing?

[Stefan Gofferje]

Hi,

could one of the developers maybe give some understanding how the
drawing process of GrADS works? I have a couple of charts in mind but no
idea how to realize them because I don't understand how GrADS handles
the parameters. Especially interesting for me is how to manipulate the
output of one parameter depending on the values of another in map
projections but also line and bar outputs.

One example: Let's say we have gxout bar. E.g. the GFS model has one
parameter for the precipitation rate as mm and a couple of parameters
which indictae the precip type and can be 1 or 0, like CRAIN, CSNOW,
CICEP...
How would I draw bars which height indicates the precip strenght but the
color changes like blue for rain, cyan for snow, red for ice pellets and
such?

If I draw on a map, e.g. gxout grid, is there a possibility to draw own
stuff, like e.g. take the temperature and if it is below zero, draw an
exclamation mark at this grid position? How would I do this? Can I
define ownfunction and then just say set gxout grid; d ownfunction? How
would ownfunction get the data to process and return what to be drawn?

Tervehdys Suomesta,
Stefan Gofferje

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