This Autoscale ability is used with some of the Chart Object scaling methods.  When the methods where Autoscale is used, that feature will set the margin area at both ends of the selected axis identified in the function being used.  The value of the axis doesn't matter.  This feature will automatically set the scale of the axis based upon the series value assigned to the custom chart elements being used for display on the chart.

 

Values assigned when an Autoscale method is use can create an upper and lower boundary range where the plotted items will be placed.  By establishing the area boundary above and below the plotting area, the minimum and maximum plotted items will not be placed up against the boundaries of the chart.  In simple terms, the custom chart areas of the axis boundaries will show some space between the plotted items and the edge of the chart.   Consider this example: Suppose a bar chart where the longest bar is 10 units.  When the chart's Y-Axis is scale the result show a 0 - 10 as the range of value.  In this example, the longest bar will touch the top edge of the plot area.  In most cases, the chart will look better with some margin at the top so that the longest bar does not touch the top edge of the chart.
 
The values use with an AutoScale method determines margin space reserved by the topExtension  and bottomExtension  arguments.  These arguments determine the portion of the axis where no data point can reach.  i.e. a topExtension of 0.2 will ensure no data point can fall within the top 20% of the axis.
 
For a purely positive axis, the bottom end has a "Zero Affinity".  That means, ChartDirector will tend to choose 0 as the bottom end because zero is a natural starting point for the axis. However, if the data range is too extreme (e.g. the data is in the range 10000 - 10005), it may be "unreasonable" to choose 0 as the axis starting point. In this case, ChartDirector will not use 0 as the axis starting point.
 
ChartDirector will determine that it is "unreasonable" to use 0 as the axis starting point if the data fluctuation (the difference between the maximum and minimum data values) is too small compare with the data value. ChartDirector test the "too small" condition using the formula:
 
maxDataValue * zeroAffinity < minDataValue
where zeroAffinity by default is 0.8.

 

     

 

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