showingTooltipIndicators property
You can show some tooltipIndicators (a popup with an information) on top of each LineChartBarData.spots using showingTooltipIndicators, just put line indicator number and spots indices you want to show it on top of them.
An important point is that you have to disable the default touch behaviour to show the tooltip manually, see LineTouchData.handleBuiltInTouches.
Implementation
final List<ShowingTooltipIndicators> showingTooltipIndicators;