Stata The Stata listserver
[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]

st: -egen- function -axis()- bundled with -tabplot- (graphics implications)


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: -egen- function -axis()- bundled with -tabplot- (graphics implications)
Date   Wed, 18 Feb 2004 13:06:42 -0000

In a separate posting, I announced the updating of -tabplot- on
SSC. 

-tabplot- uses a new unofficial -egen- function which has other
possible uses, especially for those who wish to devise their own
graphs. What follows has relevance especially for those

* using Stata 8 
* sometimes wishing to go outside the repertoire of official graph 
  types 
* accustomed to a little do-file work 

and so others may wish to bail out now. 

=======================================

The new function is called -axis()-. It is installed if you
install -tabplot-. I should, sooner or later, document it properly
somewhere, say within -egenmore- on SSC, but what follows is too
lengthy for a standard help file. 

It is easiest to explain what -axis()- does by example. Indeed,
once examples are understood, it is trivial. 

With the auto data, 
  
. egen axis1 = axis(make) if foreign 

produces this 

. l axis1 make if foreign

     +---------------------------------+
     |          axis1   make           |
     |---------------------------------|
 53. |      Audi 5000   Audi 5000      |
 54. |       Audi Fox   Audi Fox       |
 55. |       BMW 320i   BMW 320i       |
 ... 
 72. |      VW Rabbit   VW Rabbit      |
     |---------------------------------|
 73. |    VW Scirocco   VW Scirocco    |
 74. |      Volvo 260   Volvo 260      |
     +---------------------------------+

and underneath the labels, -axis1- is just integers 1/22. 

. l axis make if foreign, nola

     +------------------------+
     | axis1   make           |
     |------------------------|
 53. |     1   Audi 5000      |
 54. |     2   Audi Fox       |
 55. |     3   BMW 320i       |
 ... 
 72. |    20   VW Rabbit      |
     |------------------------|
 73. |    21   VW Scirocco    |
 74. |    22   Volvo 260      |
     +------------------------+

You are correct if this reminds you of the results of the official
-egen- function -group()- with the -label- option. -axis()- sorts
on the varlist specified and assigns integers 1 up to the distinct
values, and by default assigns value labels according to the 
varlist specified. 

There are twists to -axis()- that distinguish it from -group()-. 

. egen axis2 = axis(mpg make) if foreign , label(make) 
(52 missing values generated)

. sort axis2

-axis()- now sorts on -mpg- (breaking any ties by -make-), but the
labels are entirely from -make-: 

. l axis2 mpg make if foreign

     +---------------------------------------+
     |          axis2   mpg   make           |
     |---------------------------------------|
  1. |    Peugeot 604    14   Peugeot 604    |
  2. |      Audi 5000    17   Audi 5000      |
  3. |      Volvo 260    17   Volvo 260      |
  ...
 20. |     Datsun 210    35   Datsun 210     |
     |---------------------------------------|
 21. |         Subaru    35   Subaru         |
 22. |      VW Diesel    41   VW Diesel      |
     +---------------------------------------+

. l axis2 mpg make if foreign, nola

     +------------------------------+
     | axis2   mpg   make           |
     |------------------------------|
  1. |     1    14   Peugeot 604    |
  2. |     2    17   Audi 5000      |
  3. |     3    17   Volvo 260      |
  ...
 20. |    20    35   Datsun 210     |
     |------------------------------|
 21. |    21    35   Subaru         |
 22. |    22    41   VW Diesel      |
     +------------------------------+

Now we are in a position to use the new axis variable as one axis
of a graph, hence the name. For example, if you want a display
similar to, but a little different from, that of -graph hbar- or
-graph dot- you can make your own without any programming: 

. twoway spike mpg axis2 if foreign, hor 
	yla(1/22, noticks valuelabel ang(h)) 
	xsc(r(0,.))  ytitle(" ") 

Two further twists are shown by the results of 	

. egen axis3 = axis(for rep78), gap
(5 missing values generated)

The new labels run 

"Domestic 1" 
         "2" 
	   "3"
	   "4"
	   "5"
 "Foreign 3"
         "4"
	   "5" 

which look not too bad when set out horizontally. To put it
another way, the labels are _not_ just a concatenation of the
labels or values of the variables specified, say 

"Domestic 1"
"Domestic 2"
"Domestic 3" 
...

which, at least to my taste, looks fairly awful on a graph axis.
(They are also not as nice as what -graph bar-, -graph hbar- and
-graph dot- do by themselves. But beggars can't be choosers.) 

Also, that -gap- option implies that the underlying integers are 

  +-------------------------+
  | foreign   rep78   axis3 |
  |-------------------------|
  |       0       1       1 |
  |       0       2       2 |
  |       0       3       3 |
  |       0       4       4 |
  |       0       5       5 |
  |-------------------------|
  |       1       3       7 |
  |       1       4       8 |
  |       1       5       9 |
  +-------------------------+

i.e. there is a gap between 1/5 and 7/9 which would show up on 
a graph axis. 

In short, -axis()- is a tool for "do-it-yourself" categorical axis
variables, after which some -twoway- graph to your design is one
step closer. Customising confidence interval or similar displays
is one class of examples. 

There are more details, but if you're interested they will
become apparent on scrutiny of _gaxis.ado. 

Nick 
n.j.cox@durham.ac.uk 

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/



© Copyright 1996–2014 StataCorp LP   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index