Adding a panel

Add a panel

Add a second panel to the chart which contains a line graph with close value.

final chart = GChart(
    panels: [
      GPanel(
        valueViewPorts: ...
        valueAxes: ...
        pointAxes: ...
        graphs: ...
        tooltip: ...
        // set the height weight of the panel to 0.7 (70% of the chart height).
        heightWeight: 0.7,
      ),
      // the second panel with its own valueViewPorts, axes, graphs and tooltip.
      GPanel(
        valueViewPorts: [
          GValueViewPort(
            valuePrecision: 2,
            autoScaleStrategy: GValueViewPortAutoScaleStrategyMinMax(
              dataKeys: ["high", "low"],
            ),
          ),
        ],
        valueAxes: [
          GValueAxis(),
          GValueAxis(id: "price2", position: GAxisPosition.start),
        ],
        pointAxes: [GPointAxis()],
        graphs: [
          GGraphGrids(),
          GGraphLine(valueKey: "close"),
        ],
        tooltip: GTooltip(
          position: GTooltipPosition.followPointer,
          followValueKey: "close",
          followValueViewPortId: "",
          dataKeys: ["close"],
        ),
        // set the height weight of the panel to 0.3 (30% of the chart height).
        heightWeight: 0.3,
      ),
    ],
);

The result

chart view

On the chart all actions can be performed on both panels, such as zooming, panning, and hovering to see the tooltip.
Also you can drag the line between the two panels to resize them.