{smcl}
{* 22jun2004}{...}
{hline}
{center:{hi:Bar? graphs}}
{hline}

{p 4 4 4}{it}
Going even further into the the system on which {cmd:graph} is built, let's
see how it is possible to add a whole new graph family to {cmd:graph} and have
{cmd:graph} use that family as its own.  Official graph families, if you
recall, include {cmd:twoway}, {cmd:bar} and {cmd:hbar}, {cmd:box} and
{cmd:hbox}, {cmd:dot}, and {cmd:matrix}.  Families are the word that follows
{cmd:graph} in a graph command.

{p 4 4 4}{it}
We don't want to write lots and lots of code, so we will base our new family
on an existing family {c -} {cmd:bar}.{sf}

{hline}

{p 0 2 0}A secret about {cmd:graph}{p_end}

{p 4 6 0}	o designed to find properly named classes and use 
		  them just like official graph families {p_end}

{p 4 6 0}	o the proper name is {it:<our_family>}{cmd:graph_g.class}{p_end}

{hline}

{p 0 22 0}{view balbargraph_g.class} {c -} a class derived from the
	 official bar graph class

{p 4 4 4}{it}
We will call our new family {cmd:balbar} and derive it from the existing class
that defines {cmd:bar} graphs.  If you will click on the link 
{view balbargraph_g.class} above (click the viewers {cmd:Back} button to
return), you will see the entire contents of the class-file that defines our
new graph family.{sf}

{p 4 4 4}{it}
We won't go into too much detail about {cmd:balbargraph_g.class}, but there
are a few things to note.  At the top of the file we see a {cmd:class}
statement that defines what member variables are included in our class and
some other aspects of the class.  In this case, we see that no new member
variables are defined, but that the our class does inherit from the
{cmd:bargraph_g} class {c -} the class that defines {cmd:bar} graphs.
Inheriting from {cmd:bargraph_g} means that our new class has all of the
member variables and member programs defined by {cmd:bargraph_g}.  Because no
new variables are given in the class definition, we are not adding any member
variables.  

{p 4 4 4}{it}
This concept of inheritance allows us to leverage the capabilities of the
{cmd:bargraph_g} for our own purposes.  While our {cmd:balbargraph_g.class}
file is a scant 57 lines long, the official {cmd:bargraph_g.class} is over
3,600 lines long and implements pretty much all of the features of 
{cmd:graph bar}

{p 4 4 4}{it}
Looking back at {view balbargraph_g.class} we see that it defines a single
member program {cmd:draw_bar}.  In fact, it is redefining the {cmd:draw_bar}
graph from the {cmd:bargraph_g.class} and is thus changing how bars are drawn.
Every other feature of our new class is inherited without change from
{cmd:bargraph_g.class}

{p 4 4 4}{it}
For more documentation on classes, see {bf:[P] class} or in help
{help class}.

{p 4 4 4}{it}
We won't discuss the details of {cmd:draw_bar}.  Essentially, it uses
low-level {cmd:gdi} calls to draw lines, points, symbols, and text.  These
calls are are similar to the {cmd:gph} commands from pre-Stata8 graphics.
There are a few tricks relating to scaling because of the way symbols are
used, but most of the calls simply define how things look then draw the things
{c -} lines and such.  The entire purpose of {cmd:draw_bar} is to draw a
single bar given an {cmd:x} and {cmd:y} location for the middle of the bottom
of the bar and a {cmd:size} for the height.{sf}

{p 4 4 4}{it}

{hline}

{p 4 4 4}{it}
Let's draw a simple bar chart using an extract from the US National
Longitudinal Survey of Women from 1988.  We will use only a subset of
occupations so the graph is not too cluttered and the {cmd:asyvar} option so
that we get a little color.  (Click the next 3 commands.){sf}

{p 0 2 0}. {stata sysuse nlsw88 , clear}

{p 0 2 0}. {stata keep if occ==3 | occ==5 | occ==6 | occ==8 | occ==11}

{p 0 2 0}. {stata graph bar wage , over(collgrad) asyvar over(occupation)}

{hline}

{p 4 4 4}{it}
Now, let's get a look at our new graph family in action, by clicking in
sequence on the following command.  Note that the only change in the command
is switching {cmd:balbar} for {cmd:bar}.
{sf}

{p 0 2 0}. {stata graph balbar wage , over(collgrad) asyvar over(occupation)}{p_end}

{hline}

{p 4 4 4}{it}
We claimed that our new family inherited a host of features from
{cmd:bargraph_g.class}.  Let's see if that is true.

{p 4 4 4}{it}
Let's add the {cmd:percent} and {cmd:stack} options to our command.{sf}

{p 0 2 0}. {stata graph balbar wage , over(collgrad) asyvar over(occupation) percent stack}{p_end}

{p 4 4 4}{it}
We can indeed draw stacked percentage balloon graphs.

{p 4 4 4}{it}
What if we were to draw our graph in a high wind.{sf}

{p 0 2 0}. {stata graph balbar wage , over(collgrad) asyvar over(occupation) horizontal}{p_end}
{hline}

{p 4 4 4}{it}
Because we are looking at labor data, let's try a another {cmd:draw_bar}
program.  The {view stbargraph_g.class} also inherits from
{cmd:bargraph_g.class}, but draws a slightly more complicated "bar".  Click on
the class file if you want to see its contents, then click on the two
{cmd:graph stbar} commands.{sf}

{p 0 21 0}{view stbargraph_g.class} {c -} another class derived from the
	 official bar graph class

{hline}

{p 0 2 0}. {stata graph stbar wage , over(collgrad) asyvar over(occupation)}

{p 0 2 0}. {stata graph stbar wage , over(collgrad) asyvar over(occupation) percent stack}{p_end}


{hline}
{center:{view rip_plot.smcl:<<}   {view tindex.smcl:index}   {view arrows.smcl:>>}}