Add average line and data point lines to line charts

[agustka]

Adding extra line data to line charts which include an average line and data point lines. Also added LineStyle class and methods to draw dashed lines.

[imaNNeo]

Hi, as we talked together, I said that don't add any LineStyle, because we have to add it in another phase because borders and grids should use this too for consistency.
and I explained you completely another approach to achieve that,
let me explain it again here because these are not feet to the library codebase.
we should design it as a generic design, that possible to implement any type of use cases not just the avg line.

I explain my idea once again.
first of all lets have a base class for our line (in your example for avg line)

class FlLine { // this is a base class for our lines, (then we can use it on border and grids), should define on the base_chart_data.dart
 final Color color;
 final double strokeWidth
}

// these classes are just usable on LineCharts (or maybe BarChart or All Axis base charts),
// and you need it to draw your avg Line, or any other use cases.

classs VerticalLine extends FlLine { //let’s define this and next class on axis_chart_data.dart, because we can use them later.
  final  double y;
}

class HorizontalLine extends FlLine {
final double x;
}

// then we add it to LineChartData
class ExtraLinesData {
 final List<HorizontalLine> horizontalLines;
 final List<VerticalLine> verticalLines;
}

/ already we have this class
class LineChartData {
 …
 final ExtraLinesData extraLinesData;
 …
}

all of these was for support the average line, and for below lines that you want, we can reuse BelowBarData like this:

// already we have this class
class BelowBarData {
 …
 FlLine lineBelowSpot; // this is a line that drawn from the spot’s x and minimum y position to the spot’s x and y.
 …
}

now it is a generic design, and we can implement lots of use cases.