Crosshair
The Crosshair is a pointer represented by two mutually crossing lines stretched over the entire chart plot and helps you identify the values of the series points more precisely. When the Crosshair is enabled, its center snaps to hovered points.
To enable the Crosshair, set the chart’s BootstrapChart.CrosshairEnabled property to true.
<dx:BootstrapChart ID="BootstrapChart1" runat="server" CrosshairEnabled="true" ...>
...
</dx:BootstrapChart>