Thematic Formatting Example

An example of Thematic Formatting is automatically changing the size, color or style of points based on the contents of a data field for each point.

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Suppose we have a drawing with various city points. The illustration above shows our drawing of points as a layer in a map with a background map of the United States. Suppose the city points drawing's table has a Population field that gives the population for each city. We would like to change the size of points based on the contents of the Population field for each point.

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We begin by clicking on the point size button in the format toolbar and then choosing Theme from the menu. This launches the thematic formatting dialog.

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In the thematic formatting dialog, we choose Population as the controlling field. For this example, we use the Equal Count method so that there are an equal number of points in each of the intervals defined by the thematic format. We've also used alignment to 4 digits and pressed Tally so that the interval breaks occur at even 10000 values.

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By default, each interval has a size of 3 for the point sizes. The other numbers show how many of our cities fall within each population bracket. We can change the point size for each interval by double clicking into each size number and changing it.

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In this illustration, we've changed half of the values. We've chosen a progression of sizes that we think will be a reasonable increase in size for each interval. For example, the cities with populations from 20000 to 30000 will have a size of 4. If this turns out not to provide the desired visual effect we can open this dialog again and adjust the values.

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We've now changed all the values. The interval from zero to the first number has a size of 1 and the interval from the last number up has a size of 10.

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When we press OK the points will be redrawn with size values corresponding to the contents of their Population field. Points with larger Population will be drawn as larger size as specified in the thematic format.

Changing Color

We can also change color using a thematic format. In fact, we can change color with a thematic format even as we change size. To change color, we click on the background color well and choose Theme to launch the thematic format dialog.

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In the thematic format dialog we once again choose Population as the controlling field and Equal Count as the method to use. Once again, we use an alignment to 4 digits and press Tally to get intervals of even 10,000. When creating thematic formats for colors, Manifold provides a choice of Palettes as well. Each palette provides a handy color range. We choose the Blues to Greens palette.

$images\btn_theme_apply.gif$ We press Apply to apply this palette to the intervals used.

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The palette applies a range of graduated colors from blue to green to the intervals in our thematic format. We then press OK.

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The points will now be drawn with color that varies by the value of their Population field. Note that the color change is in addition to the change in size for each point. This provides greater emphasis on the more populated cities.

Changing Style

If desired, we can even vary point style with a thematic format. To do so, we click on the point style button in the format toolbar and choose Theme. This launches the thematic formatting dialog for varying point style. Once again, we will use Population as the controlling field and Equal Count as the method for assigning intervals. We will also choose an alignment to 4 digits and press Tally to get intervals of even 10,000.

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This time, when we double-click into each interval's attribute we will change the style instead of changing a size value or color. The dialog opens with all circles. We can double click on the circle for each interval and change the circle to the desired style. In this case, we have specified a series of styles from a simple cross to a circle to a square and so on to an inverted triangle.

We press OK to apply the thematic format.

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The points will now be drawn with a style symbol that varies in accordance with the Population field. Note that the style variation is in addition to the color and size variation specified in the other thematic formats we applied.

Tips

When applying more than one thematic format, it is almost always best to use the same method and the same number of intervals for all thematic formats. It's too much to ask of viewers to expect them to keep in mind that size varies in accordance with five intervals but color varies only by three intervals.

It is not easy to visualize three different variables at once even when they are each coded by size, color or style. Use multiple thematic formats with great restraint. Note how in the above examples the image with circles of varying size but all of the same color does a good job of conveying the relative populations of the towns. There is some value added in adjusting color and style in some circumstances; however, the more frequent case is that overuse of thematic formats to display many different attributes at once will lead to a confusing presentation.

$images\btn_theme_interpolate.gif$ A fast way to assign a range of values is to use the Interpolate button. Click on the topmost interval and assign the value (such as color or size) for that interval. Next, click on the bottom interval and assign the value for that interval. Finally, click the Interpolate button to fill in the intervals in between with interpolated colors or values.

When interpolating styles, the intermediate intervals will be filled in with the symbols that occur, in order, in the styles menu from the first interval's symbol to the last interval's symbol.