added gradient color to range annotations, issue #1195.

[MagdyYacoub1]

Resolving issue #1195

Added gradient attribute to horizontalRangeAnnotations and VerticalRangeAnnotation.

This will help specify gradient or solid color to range annotations. If none were identified, it would use a solid whit color.

[imaNNeo]

Ready to merge, but before that please update the line_chart docs

[codecov]

Codecov Report

Merging #1294 (02a3bc6) into master (68917b0) will decrease coverage by 0.04%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master    #1294      +/-   ##
==========================================
- Coverage   85.94%   85.90%   -0.04%     
==========================================
  Files          45       45              
  Lines        2945     2958      +13     
==========================================
+ Hits         2531     2541      +10     
- Misses        414      417       +3     

Impacted Files	Coverage Δ
lib/src/chart/bar_chart/bar_chart.dart	0.00% <0.00%> (ø)
lib/src/chart/line_chart/line_chart.dart	0.00% <0.00%> (ø)
lib/src/chart/scatter_chart/scatter_chart.dart	0.00% <0.00%> (ø)
.../src/chart/base/axis_chart/axis_chart_painter.dart	72.51% <45.28%> (+1.36%)	⬆️

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[MagdyYacoub1]

Thank you for your reply.
I updated the documentation of the horizontalRangeAnnotations and VerticalRangeAnnotation by adding the gradient attribute to their tables.
Also, I updated the comments in the ''axis_chart_data.dart'' to explain more the color and gradient attributes behavior when both provided or not provided.

[imaNNeo]

Great!
Thanks for contributing.