A chart must be connected to a data source, which can be any array in the report's data. Set the data source in the chart element's properties panel while the chart is selected in the designer. The drop-down will show available arrays in the data source, based on the sample data.
Once a chart has an array of data to draw values from, you should specify which values are data values and which are labels. For line and bar charts, the value field provides the y-axis value, that is, how high the bar or point on the line is vertically. The label field provides the label shown on the x-axis along the bottom.
For pie charts, the value field sets the relative size of the slice, and the label field sets the name of the slice shown in the chart legend.