FlChart to display different time series - firebase

Question

I try to use the following code for my application to display the weight of animals in a chart. My goal is a variable time controlled via buttons (e.g. 1 month, 6 months, 1 year,...).
LineChart(
LineChartData(
lineTouchData: LineTouchData(enabled: true),
gridData: FlGridData(
show: false,
drawHorizontalLine: false,
drawVerticalLine: false,
getDrawingVerticalLine: (value) {
return FlLine(
color: Colors.black,
strokeWidth: 1,
);
},
getDrawingHorizontalLine: (value) {
return FlLine(
color: const Color(0xff37434d),
strokeWidth: 1,
);
},
),
lineBarsData: [
LineChartBarData(
spots: [
FlSpot(0, data_snap()['Historie_Inhalt']['0']),
FlSpot(1, data_snap()['Historie_Inhalt']['1']),
FlSpot(2, data_snap()['Historie_Inhalt']['2']),
FlSpot(3, data_snap()['Historie_Inhalt']['3']),
FlSpot(4, data_snap()['Historie_Inhalt']['4']),
FlSpot(6, data_snap()['Historie_Inhalt']['5'])],
isCurved: true,
barWidth: 1,
colors: [
Colors.black87,
],
belowBarData: BarAreaData(
show: true,
),
aboveBarData: BarAreaData(
show: true,
colors: [Colors.grey.withOpacity(0.3)]
),
dotData: FlDotData(
show: false,
getDotPainter: (spot, percent, barData, index) =>
FlDotCirclePainter(
radius: 10,
color: Colors.deepOrange.withOpacity(0.5)),
),
),
],
titlesData: FlTitlesData(
bottomTitles: SideTitles(
showTitles: false,
reservedSize: 10,
getTitles: (value) {
switch (value.toInt()) {
case 0:
return '2019';
case 1:
return '2020';
case 2:
return '2021';
case 3:
return '2022';
default:
return '';
}
}),
leftTitles: SideTitles(showTitles: true, margin: 3,
reservedSize: SizeConfig.safeBlockHorizontal * 11
),
rightTitles: SideTitles(showTitles: false),
topTitles: SideTitles(showTitles: false),
),
),
swapAnimationDuration: Duration(milliseconds: 1500),
)
I get the following error directly for the first FlSpot:
type 'String' is not a subtype of type 'int' of 'index'
So far I retrieved only six fixed values from Firebase and displayed them with the following code:
FlSpot(5, data_snap()['P_1W_W']),
'P_1W_W' is in the screenshot below on the same level as 'name'.
The database looks like this ('Historie_Inhalt' is defined as an Array):