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BarChart - Extendng the Y-axis and labeling the endpoints

• To: mathgroup at smc.vnet.net
• Subject: [mg97536] BarChart - Extendng the Y-axis and labeling the endpoints
• From: Donald DuBois <donabc at comcast.net>
• Date: Sat, 14 Mar 2009 18:16:30 -0500 (EST)

Here is a BarChart (almost directly  from the Mathematica Documentation):

Needs["BarCharts`"]

thisBarChart = BarChart[{1, 2, -1.3, 2.4}]

I am trying to extend the labeling of the Y-axis so that the endpoints of the Y-axs extend a little beyond
the extremes of the data  in the Y direction (the extremes of the data
being -1.3 and 2.4) and these endpoints are labeled with
multiples of the chosen increment. 

I believe this makes the BarChart easier to read and is aesthetically more pleasing.  

For example, with the above
evaluation of BarChart,  the labeling of the Y-axis  goes from 2.0 to -1.0 in increments of 0.5 as chosen by Mathematica but the Y-axis itself extends
beyond these labels. 

  I would like to extend the labeling in both directions (positive and negative sides
 along the Y-axis)  to the nearest integral multiple of 
  the increment that Mathematica has chosen so the Y-axis
  would be extended slightly beyond the extremes of the data and the endpoints of the Y-axis would have labels.

In this example,  the Y-axis would be extended from 2.5 to -1.5 and the labeling would go from  

{ -1.0, -0.5, 0.0, 0.5, 1.0, 1.5, 2.0} to  

{-1.5, -1.0, -0.5, 0.0, 0.5, 1.0, 1.5, 2.0, 2.5} 

with the labeling being integral multiples of the chosen increment.   
   
To do this, I need to know that the automatic labeling is in increments of 0.5 (as chosen by Mathematica) and then I need to extend the endpoint labels from {-1.0, 2.0} to  {-1.5, 2.5}. 

I can get the PlotRange that is chosen by Mathematica   automatically by using  AbsoluteOptions on the graphical object called thisBarChart: 

AbsoluteOptions[thisBarChart, PlotRange]   

which produces

{PlotRange -> {{0.4, 4.6}, {-1.3, 2.4}}}

But, I still don't know the increment that Mathematica used so that I can force the labeling to a high of 2.5 and a minimum of -1.5.

There's another complication:

Assuming I did know (by magic) that the chosen increment was 0.5 for the labeling I thought I could then use

BarChart[{1, 2, -1.3, 2.4}, PlotRange -> {Automatic, { 2.5, -1.5}}]

to extend  the Y-axis endpoints with labels of -1.5, and 2.5  but this doesn't work because the endpoints of the Y-azis are not labeled.

I find, through experimentation only, that what I need is:

BarChart[{1, 2, -1.3, 2.4}, PlotRange -> {Automatic, {3.0, -2.0}}]

and the increment for the labeling has automatically changed to 1  (the fact that the increment has changed from 0.5 to 1.0 in this case is not a problem.)

How can I get Mathematica to do this in a programmatic way so that I don't need to experiment each time to (1) get the Y-axis extended  a little
beyond the extremes of the data  and (2) the endpoints 
labeled with integral multiples of the chosen increment?  

Thank you in advance for any help you can give me.