Suggestions for improving plots

[Billy Kessler <kessler@xxxxxxxxxxxxx>]

Hello Ferret users -

I see a lot of people using Ferret who always rely on the default 
plot styles. That is fine for the quick-look, but with a very small 
effort using the PPLUS commands, Ferret plots can be made professional-
looking, which is to say uncluttered and easily readable. In fact that 
is one of Ferret's main attractions, as far as I'm concerned.

We spend so much time preparing our data or model output, it is 
well worth the small amount of additional effort to clean up the 
plot so the results shine through clearly. 

Following are a few suggestions to correct common problems.
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Problem 1: Axis tics and labels.
By default, Ferret uses all large tics and labels every other tic. 
For many purposes, that is not ideal. Better to have the large tics 
all labeled, and use small unnumbered tics for the smaller intervals. 
These functions are controlled by:

PPL AXLINT,xint,yint	    ! how often to label tics (default = 2)
PPL AXNMTC,numticx,numticy  ! how many small tics between each large tic (default = 0)

Both of these functions are persistent. Once set, they remain in 
force until changed. Full use of this capability is achieved by 
also specifying the axis limits in the PLOT or FILL command:

PPL AXLINT,1,1
PPL AXNMTC,2,4
FILL/HLIMITS=120w:70w:30/VLIMITS=20s:20n:5 variable

In this example longitudes are ticked and numbered every 30 degrees, 
and latitudes every 5 degrees. There are 2 tics between each longitude 
label and 4 between each latitude label. Not so hard and looks much 
better.

It is also easy to change the size of the tic numbers:

PPL AXLSZE,xsize,ysize

This is also a persistent command that only needs to be given 
once/session.

A final peeve of mine is leaving too many digits in axis numbers. 
Ferret will often add an extra zero after the decimal in axis numbers,
and it will include the decimal point following integer values. 
These clutter the plot and distract the reader from the data. 

To fix this specify the axis formats with PPL XFOR, PPL YFOR.
These commands are not persistent and must be given for each plot:

PLOT/SET variable
PPL XFOR,(i3)		! as appropriate
PPL YFOR,(f5.2)		! as appropriate
PPL PLOT

Note that if the data grid is not longitude or latitude but you want 
to force the labels to be geographic:

PPL XFOR,(i5,''lon''); PPL YFOR,(i5,''lat'')

In fact you can add any extra text to axis numbering:

PPL XFOR,(f4.1,''text1''); PPL YFOR,(i3,''text2'')

Suppose you are plotting temperature on the y-axis and want 
degree marks on the axis numbers:

PPL YFOR,(i2,''#'')

Finally, sometimes the axis titles are not right. 
Add these manually:

PLOT/SET variable
PPL XLAB,This text will appear as an axis title
PPL YLAB,And this text as a y-axis title
PPL PLOT

Control the size of these titles with LABSET:

PPL LABSET,main-title-size,x-label-size,y-label-size
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Problem 2: Too many digits in contour labels.

As with axis labels, Ferret will often include an extra zero after 
the decimal point in contour labels, or use a superfluous decimal 
point. These take up valuable reader attention by cluttering the plot.
Fix these by specifying the contour labels manually. Usually you would 
first do a default plot to see what the contour range is:

CONTOUR variable	! note the contour range
CONTOUR/LEV=(lowerlev,upperlev,interval,flag) variable

The option "flag" tells how many decimal places to use: 0-9 for the 
number of decimal places, -1 for integer labels, -3 for no labels.

Also note that several sets of levels can be specified for total control:

CONTOUR/LEV=(-3,3,1,-1)(-.5,.5,1,1)del(0) variable

This example makes integer-labeled contours for -3,-2,-1,1,2,3, 
single-decimal-place-labeled contours for -.5,.5 and omits the 
zero line.
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Problem 3: Color key labels too small.

The default is fine for a quick-look, but will sometimes be too small 
for good readability. Fix this with PPL SHAKEY:

PPL SHAKEY,,,label-size-in-inches

Note that PPL SHAKEY has many other options, which allow you to specify 
many things about the color key:

PPL SHAKEY [0=nokey,1=key] [0=hor,1=vert] lab-size lab-inc lab-dig lab-len xlo xhi ylo yhi

One way these are useful is if there are, say, a stack of three plots 
all with the same contour interval. It is much clearer in that case 
to have a single color key, extending the length of the stack:

SET VI top
FILL/NOKEY/LEV=(level-specifiers) variable1

SET VI middle
FILL/NOKEY/LEV variable2   ! LEV without arguments repeats the previous levels

SET VI bottom
FILL/SET/LEV variable3
PPL SHAKEY,,,,,,,`1.2+($ppl$xlen)+.4`,`1.2+($ppl$xlen)+.7`,1.4,`1.4+3*($ppl$ylen)+1.`
PPL FILL

The last 4 options on SHAKEY specify the x-coordinates of the left
and right side of the color key, and then the y-coordinates of the 
bottom and top of the key, relative to the page origin. The plot 
origin is (1.2,1.4), the axis lengths are ($ppl$xlen) and ($ppl$ylen), 
and the final value in the above example (1.) is a guess at the 
extra space between the three plots. You have to find this by 
experiment. This is not something you would do for an exploratory 
plot, but is worth the effort for a plot to be shown or published.
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Problem 4: Equator line missing.

All maps need an equator line! In almost every case, equatorial 
dynamics have a small meridional scale so it is highly relevant 
whether a signal is exactly at the equator or slightly north and 
south. This is the simplest of all:

PPL CROSS,1

Now you will always have an equator line on your maps.
This is a persistent command, so just give it at the beginning of 
a Ferret session, or put it in a startup script. PPL CROSS,0 cancels.
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Billy K