Please describe the problem:
The metapost command label uses a bouding box as a reference point for 
positioning text.  This makes consistent positioning impossible.

Steps to reproduce:
1. Create a UML class diagram
2. Enter a list of properties, some with tall letters T,h,I, some with normal 
letters e,o,u,w, and some with descending letters p,y,j.  Say This, puny, car, 
with some other words thrown in for good measure.
3. Export MetaPost, compile, and observe the uneven spacing of the words.


Actual results:
The words aren't spaced right.

Expected results:
Have them be spaced right

Does this happen every time?
Yes

Other information:
> It turns out that MetaPost positions text based on the bounding box of
that
> particular text element.  So, the left, right, top, bottom, etc. commands
> are difficult to apply correctly.  I.E. if you position the words: dog cat
> sea This puny.  And you try to align them by their tops, bottoms, or
> middles, you'll get a zigzag of words.  Some poke down, some poke up, some
> do both, and some do neither....

As the label command (and all its variants) are based on positioning text by
the coordinates of its bounding box, it is impossible to get precisely
controlled text layout (using that command) in MetaPost.

Instead, you have to use the (less powerful) draw command...  It is less
powerful because draw won't take care of left, right, center positioning for
you.  You'll have to calculate that yourself...

For an example of a list of attributes in a UML diagram.

So, instead of this:

label.urt(btex {\usefont{OT1}{cmr}{m}{n} dog} etex scaled 1.0,
(0.0x,0.00y));
label.urt(btex {\usefont{OT1}{cmr}{m}{n} cat} etex scaled 1.0,
(0.0x,0.35y));
label.urt(btex {\usefont{OT1}{cmr}{m}{n} sea} etex scaled 1.0,
(0.0x,0.70y));
label.urt(btex {\usefont{OT1}{cmr}{m}{n} This} etex scaled 1.0,
(0.0x,1.05y));
label.urt(btex {\usefont{OT1}{cmr}{m}{n} puny} etex scaled 1.0,
(0.0x,1.40y));

We need to do this...

draw btex {\usefont{OT1}{cmr}{m}{n} dog} etex scaled 1.0 shifted
(5.0x,0.00y);
draw btex {\usefont{OT1}{cmr}{m}{n} cat} etex scaled 1.0 shifted (5.0x,
0.35y);
draw btex {\usefont{OT1}{cmr}{m}{n} sea} etex scaled 1.0 shifted
(5.0x,0.70y);
draw btex {\usefont{OT1}{cmr}{m}{n} This} etex scaled 1.0 shifted
(5.0x,1.05y);
draw btex {\usefont{OT1}{cmr}{m}{n} puny} etex scaled 1.0 shifted
(5.0x,1.40y);

The 'x' coordinate difference is just to separate the two columns of text.
The 'y' values are separated by 0.35cm (the font size).  Using the draw
command with the shifted transformation, you position text by the left end
of the baseline.

Using the label.??? command, you position text such that its bounding box is
positioned ??? relative to the coordinate point.  In this case, label.urt
means the bounding box is up and to the right of the specified coordinates.

While left-right-center will take some special handling (although Dia must
do this someplace already), this should be an answer once and for all...
There were some hackish fixes suggested where you insert an invisible
character that reaches to the top of the tallest and the bottom of the
lowest character.