From within the chart builder, you can see some of the data in the data set linked to the chart. Previewing sample data can help you decide which columns to use to plot the x- and y-axes. For example, in Figure 21‑27, Data Preview shows several columns of data from the Classic Models, Inc. database that ships with BIRT.
Figure 21‑27 Previewing the data in the chart builder
Data Preview serves another important purpose as you define charts. You can drag and drop the columns from Data Preview to the value, category, and optional Y grouping definitions in the upper portion of Select Data.
As you set up the expressions that specify the data for your chart, Chart Preview presents a rough idea of how your published chart will look. Figure 21‑28 shows the chart preview of a bar chart.
Figure 21‑28 Previewing chart data on Select Data
You can change some characteristics of chart previews to improve performance. As you design a chart, the chart preview uses either live data, as shown in Figure 21‑28, or randomly generated sample data, as shown in Figure 21‑29. You can also change the number of data rows that appear in the chart builder. By default, the Data Preview section shows six rows of data. You can display more or fewer rows.
Figure 21‑29 Bar chart preview using random data
How to change chart preview preferences
1 Choose Window➛Preferences.
2 Expand Report Design. Select the Chart list item. Expand Chart and Field Assist, as shown in Figure 21‑30. Field Assist and Content Assist preferences include options to flag required fields in the chart builder and to display hints for building an expression.
Figure 21‑30 Chart page of Preferences
3 To have the chart builder use randomly selected data in the chart preview window, deselect Enable Live Preview.
4 To set the number of rows that Data Preview displays, type a value in the field.
