removeLine method
Implementation
void removeLine(int y){
data.removeWhere((unit)=>unit.posGetter(axis).y == y);
var remains = data.map<GridUnit>((unit){
if(unit.posGetter(axis).y < y){
return unit.offset(GridYStep);
} else {
return unit;
}
}).toList();
data.clear();
data.addAll(remains);
}