The nine chart types

GDDM-PGF V2R1.3 Application Programming Guide

The nine chart types

GDDM-PGF V2R1.3 Application Programming Guide
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PG routines let you draw these different types of chart:

Line graphs (and scatter plots)
Surface charts
Histograms
Bar charts
Tower charts
Polar charts
Pie charts
Venn diagrams
Table charts.

Line graphs: The plotted points for each component are joined by a polyline (or, optionally, a fitted curve), and each plotted point is indicated by a marker. In general each component uses a different color, line type, and marker to aid recognition. A typical line graph is shown in Figure 15 in topic 5.1.10. A special case of the line graph is when the lines themselves are suppressed. The resultant chart is known as a scatter plot. An example is shown in Figure 12 in topic 4.6. Alternatively, the lines may be shown without markers on the plotted data points.

Surface charts: These are similar to line graphs but the areas between the successive data lines are shaded and the plotted points are not indicated by markers. A typical surface chart is shown in Figure 18 in topic 5.2.6. This is one of the so-called stacked chart types. The value of a component for a particular x coordinate is naturally interpreted as the depth of the layer at that point - in other words the difference between the y values of that component and those of the previous component. Stacked charts therefore normally use "relative data." This concept is discussed in "Absolute and relative data" in topic 3.4.

Histograms: In a histogram a particular bar represents the y data value over the range of the x axis indicated by the width of the bar. For example, in the typical histogram shown in Figure 19 in topic 5.3.2, the first bar indicates that 12.5% (the y value) of the population are aged between 0-10 (the x range, indicated by the bar width). Histograms, like the previous two chart types, have a numeric x axis. Any further components are stacked on top of the first component.

Bar charts: Here each bar (or set of bars if there is more than one component) represents a separate x entity. For example, each bar might represent the copper production of a particular country. The bars are of equal width. Often they are associated with successive days, weeks, months, or years. The bars can be regularly spaced out along the x axis without representing any significant x values, as when showing copper production by country. Or they can be placed at significant x values, as when the x axis represents time. If the x values are not significant, the chart is said to have a logical x axis. If they are, the chart is said to have a numeric x axis. If the y data represents values occurring at regular time intervals, as in the bar chart shown in Figure 22 in topic 5.4.9, then logical and numeric x axes would give similar results.

Tower charts: These are similar to bar charts, but have a three-dimensional appearance and a third axis, the z axis. They are sometimes called skyscraper or Manhattan charts. They usually have several components, each being drawn at a different specified z value, like the example in Figure 23 in topic 5.5.9. Bars can be stacked but towers cannot; otherwise, the choice between bar and tower charts depends mainly on appearance.

Polar charts: These are like circular line graphs. They are sometimes called star or radar charts. If the x axis represents a single continuous variable, then polar charts are most useful for displaying cyclical data. A patient's temperature taken at intervals over a day would be a typical example. This type of polar chart is illustrated in Figure 24 in topic 5.6. Another important use is for comparing two or more entities, such as companies, on several different parameters, such as revenue, profit, capital invested, and so on. There is an example of this type in Figure 25 in topic 5.6.1.

Pie charts: The sectors of a pie chart indicate the relative size of various elements within a whole. The pie chart shown in Figure 27 in topic 5.7.4, for example, shows that sales in the London region accounted for 16% of the 1991 sales. In a multiple pie chart, several pies are placed side by side for comparison.

Venn diagrams: This uncommon form of chart displays the logical relationship between two overlapping sets. For example, the Venn diagram shown in Figure 28 in topic 5.8.1 shows that 37 of the 113 female students entering St. Basil's College intend to study mathematics.

Table charts: This form of chart is simply data displayed in rows and columns. The table is contained within a ruled box, and ruled lines also separate the columns, the component names, and x-labels. There is a table chart at Figure 29 in topic 5.9.

All the example charts mentioned above were created using the PG routines. They might equally well have been built using the Interactive Chart Utility (ICU). The ICU translates the requests made by the operator into PG routine calls. These are subsequently translated into GDDM line, arc, and area calls.

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