Drawing Normal Graphs - Shading not consistent

[Albert Kinderman]

Hi:

I teach statistics and find it useful to draw graphs of the normal 
distribution with certain areas filled in (shaded).  I have tried 
to create a def that will produce the shaded graph.  What follows 
is my mydefs file and a sample document.

The problem is that sometimes the shading goes all the way to the 
upper limit and sometimes it stops short.  Note, because I step 
through the graph by .1, both the lower and upper arguments should 
be rounded to the nearest .1.

Does anyone have any suggestions?



[file mydefs]

import @Graph @Data

def @NormalDensity
        right x
        { {1/{sqrt 2*pi}}*e exp {-.5*x*x}}

def @NormalFilledGraph left low right high
{ @Graph
        style { axes }
        belowgap {0}
        xorigin {0}
        yorigin {0}
        xmin {-2.5}
        xmax {2.5}
        ymin {0}
        xticks {}
        yticks {}
        yextra {0}
        height {2 cm}
{       
        @Data   pairs {solid}
                paint {yes}
                color {lightgrey}
        {xloop from {low} to {high} by {.1} do
                {x @NormalDensity x}}   
        @Data   pairs {solid}
        {xloop from {-2.5} to {2.5} by {.1} do
                {x @NormalDensity x}}   
        @Data   pairs {solid}
                {low 0 low  @NormalDensity low}
        @Data   pairs {solid}
                {high 0 high @NormalDensity high}
} 
	
}


[sample document]

@SysInclude { graph }
@SysInclude { doc }

@Doc
@Text
        @Begin
@CD   .6 @NormalFilledGraph 1.4

@CD -2.5 @NormalFilledGraph .3

@CD   .3 @NormalFilledGraph .7

@CD  -.6 @NormalFilledGraph .5

        @End
@Text