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Chart Accessibility


by Björn Milcke (Bjoern.Milcke@Sun.COM),
last modification on March 5th 2002

1. Introduction

This paper describes how documents of the StarOffice/OpenOffice.org Chart application can be made accessible by using the UNO Accessibility API (UAA).

2. Chart Documents

In contrast to other applications, a chart document has a relatively fixed structure. For most objects in a chart you always have at most one instance. For example, there may only be one main title object and one legend. Only the number of data series and data points inside of series may change, which depends directly on the dimension of the input data.

There is only one page in a chart. On this page there are the following top level objects:

The page's background color is determined by the chart area object, which is the object that you can select via the GUI to format the background of the entire space a chart document occupies.

The Diagram is the object containing the actual plot. In three-dimensional charts the diagram is a scene object. There are four kinds of sub-objects in the diagram:

The number of objects we are dealing with here is limited to a small constant for all objects except the series. These can vary in number depending on the size of the input data; however usually, this number should also be small. Having more than ten, or so, series usually doesn't make much sense. In contrast, the series themselves can contain a lot of data points. This might become a problem for AT (Assistive Technology) tool. We will discuss this issue in the following section.

3. Data Series and Data Points

As stated before, the number of series usually is small. So, we assume not having problems here. However, the number of data points contained in a series may be huge, and although you may not be able to really distinguish all points from each other on a screen, they are all visible in the current window, thus have to be made accessible.

Consider a chart where you have five series, each representing 2,000 data points displayed as little symbols. You can select every of those points (e.g. via the keyboard) and give it a different formatting, thus you should also provide them as objects in the document tree for getting notified on selection. As a result we would get 10,000 objects in the tree for those objects. This will consume a lot of time and also memory. Maybe a tool might even crash when getting such a lot of nodes, because normally you do not have so many objects in a document that are all visible at the same time (especially in dialogs, for which most tools were initially designed). There are several solutions to this problem, which we want to discuss next.

Assume that AT-Tools are Clever

An AT-tool may feel free to query the number of children and accessible object has. If it turns out that this number is quite high (according to some threshold), the tool could simply not request those objects. Even on rendering, those objects could be omitted, because their bounding boxes must all lie in the bounding box of the parent element.

However, if a user travels down the document to a series, the tool would most probably load all children to offer them to the user. Alternatively the tool could just state how many children there are, and the user could select the nth child without knowing what this child looks like.

The question here mainly is, if AT-tools will be enhanced when the requirements increase for using StarOffice/OpenOffice.org. As this does not lie in our hands, this way has a certain risk.

Offer only those Data Points that have their Own Attributes

When you create a chart from scratch, all data points have no own attributes. Instead they inherit their appearance from the series. A user can select a data point and apply different properties to it. Internally, there is a list of all data points that differ from the default, i.e., have been modified by a user. The number of modified data points usually is also small.

This way, a user accessing a document via an AT-tool could get information about all data points that are differently formatted. For example, if a series has a blue fill color, and one data point is made red to underline its importance, the user would have access to it. However, the user would not be able to select an unformatted data point to apply a different color herself.

Do not offer Data Points at All

The data points are usually not formatted differently from the series. And even if some points are differently formatted, the question remains if a user is interested in that. One important property of a data point is its value. But the value can be found in the underlying data table, which is also accessible. So we could generally ignore data points.

The only downside of this is that you can select data points via the GUI, e.g., the keyboard. When doing so you would get an object that is not accessible from the view.

Offer Data Points dependent on Chart Type

If you take for example pie charts, you will notice that there will usually not be too many data points, otherwise you could not really interpret this chart visually (which is the main purpose of a chart). And also bar charts, mostly used in business graphics, do not have too many bars.

The most prominent examples for charts that may represent huge amount of data, are line charts and xy-charts (scatter charts). Those two chart types also have subtypes that contain just the lines and no symbols. In this case there simply do not exist any data points. Thus, the problematic situation is (under our previous assumptions) having a line- or scatter chart using symbols for a lot of data points.

One solution (which in fact solves the problem just under the assumptions made) would be not to offer any data points for certain chart types, namely line- and scatter charts (and maybe radar charts).

Offer Data Points dependent on the Quantity

As we can never be sure that there are no bar charts with thousands of bars, the previous approach may fail. So we could use a heuristic that children are only created, if their number is smaller than a given constant threshold. This way we would be on the safe side. We would also have no problems for all types of charts having only a small number of data points per series.

A remaining problem would still be the inconsistency, that you could select data points via the GUI (the keyboard) but would not get accessible objects from the selection. However, this seems to me the most promising approach at the moment.

4. Additional Shapes

You can add additional shapes to a chart. When being inserted via the clipboard those shapes are always meta-files or bitmaps. These shapes can be treated the same way as shapes in the Draw/Impress. In other words, we can get an XShape from the drawing layer that supports all necessary accessibility capabilities.

4. Visibility

The tree of accessible objects is always constructed of the objects that are in the current scope of the screen. Charts may only be OLE objects, and the view for an OLE object is only created when it is entered in In-Place mode. On activation the OLE-object is moved such that it is completely visible, unless its size exceeds the size of the window. This is the only (rare) situation when not all objects of a chart may be visible.

Apart from that, consider a bar chart where the rightmost bar is not in the current scope. I think it doesn't make much sense to offer all data series, but one series has one data point less than the others, just because it is not visible.

Therefore I suggest to always offer the complete tree of objects, no matter if really everything is visible.

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