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Figure 1 in Chapter 3 gave a pictorial representation of the types of chart. We shall look at the general features of each type and describe many of the terms used in the facilities matrix (see Appendix 2)
Next, we examine the details of charts and the terminology involved, The term 'data series' is used to describe a group of numbers or measurements which refer to one variable. Some packages limit the amount of data in any one series, while keeping the total amount of data constant; others set an upper limit on the number of data series while allowing each series to contain large amounts of data. These constraints are often very important in the choice of package appropriate to the analysis and presentation of the data. Always bear in mind that a chart can become overcrowded and unintelligible if there are too many data series, so some of these constraints may be beneficial.
We cannot cover every possible chart or pictorial representation as each discipline has its own specialities, but the facilities offered by packages continue to develop and expand. If a particular feature is not available today, check again in six months time - a new product may be available or the facilities offered by existing packages may have changed.
Figure 2: The anatomy of a chart
The title and subtitle, and perhaps the footnote, are common to many types of chart. Sometimes they are identified specifically and sometimes they are added as separate text, not identified specially. The title and subtitle give the audience an important 'handle' on the message and draw attention to what is being said. Sometimes they emphasise the speaker's identity or affiliation. The footnote is useful in organising charts.
The legend explains what each series of data represents. Different line types are required for black and white output, while different colours can be used to distinguish the different data series if colour output devices are available but bear in mind that colour vision is impaired in a significant number of people, and also that certain colours do not stand out in poor lighting conditions. In some circumstances it can be valuable to give dhe exact value of a data point, while in others, overall trends are all that is required.
'Missing data' are always a problem. If a data series is likely to be incomplete, how does a package respond? Does it simply skip over the blank or does it interpret absence as zero? Can missing data be coded so that the package knows to skip over the value? Will the package expect a full data series for each variable in a group?
Many series of data contain outliers or extreme values which distort the presentation: will logarithmic axes be necessary? If so, how will the package represent a zero value? Can the axes be split to avoid large areas of empty space?
Figure 3: The anatomy of axes
Examination of papers in scientific journals will show that there is a wide variety of methods for presenting axes: does the target journal have a specific style? Will separated axes be necessary? How many major divisions are appropriate for the data and what is the best number to choose to enhance the message without cluttering the display? Can the maximum and minimum values for the axes be defined? Are subdivisions really necessary or will it be best to give exact data values? Will the placing of the axes' tick marks be important?
What sort of X-data will be employed? Textual, numeric or date? If a time series is required, what is the best way to show the divisions? How will the axes be labelled? Will scientific notation, subscripts, superscripts and non-English characters be necessary? If so, can the printer produce the full range?
Is the orientation of the Y-axis label important? Can it be aligned vertically? How many X and Y axes are needed and how many are optimal to present the data, but to avoid overcrowding the image by presenting too much data? Will contours and Z axes be necessary?
Figure 4: The anatomy of bar and column charts
The line or scatter graph is very widely used for presenting scientific data, while bar and column charts are often used for showing differences between discrete variables. Many of the ideas introduced here can be applied to other types of chart.
The width can be varied so that columns touch, as they do with histograms, or points can be emphasised by keeping the columns separate. The display can be made less cluttered by overlapping the columns or by stacking the data. Percentages can be emphasised by employing the stacked column (100%) technique and some pleasing results can be obtained by adding 3D effects. The combination of 3D effects and overlapping can, in certain circumstances, hide data if a tall column comes in front of a short column; in this case it may be possible to change the angle of vision by rotating the chart. If this proves necessary, is the choice of chart type appropriate?
The paired bar chart is often good at showing differences between two categories, such as the comparative performance of two cars over a number of criteria. The choice of fill pattern can also be important P solid colour can be easier to distinguish than similar patterns, particularly if there is unfortunate juxtaposition of small data sets.
Figure 5: The anatomy of a Pie chart
A Pie-chart is a popular and useful way of representing small numbers of variables. Particular aspects can be emphasised by exploding segments. Some packages are poor at exploding 3D pie charts. Will it be necessary to show data values, or percentages of the total? If there is an aggregated group ('other'), will it be useful to have a linked pie or column chart giving a detailed analysis?
The pie chart is ideally suited to small numbers of variables, so the number of fill patterns or colours available will rarely be an issue, although the appropriate choice of pattern or colour will be important.
Figure 6: The anatomy of a Polar Chart
Polar charts are used in science and engineering in applications where it is more natural to express coordinates in polar than in cartesian form.
Figure 7: some features offreehand drawing
Figure 8: more features of freehand drawing
The issue of picture design is a potentially complex one which raises many questions, such as:
If users are uncertain about these or any other aspects of design, professional advise should be sought. Refer to the supplement for references to any such bodies within your organisation.
On the specific issue of colour in computer graphics, there are numerous publications. Some very useful introductory articles/booklets for anyone interested in learning more about colour usage are:
The specific topic of colour blindness, and how to produce pictures which compensate for it, is covered in:
Local advice and access to reference documentation may be available locally - refer to the Supple nent at the end of this document
Graphics Multimedia Virtual Environments Visualisation Contents