Added drawGridLinesOnTopEnabled boolean flag to draw grid lines on top of Bar Charts

[JacopoMangiavacchi]

I had this UI requirements for a projects and added this boolean flag, and relative behaviour, to draw the grid lines above the Bar Charts

[codecov-io]

Codecov Report

Merging #2390 into master will increase coverage by 0.01%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##           master    #2390      +/-   ##
==========================================
+ Coverage   19.66%   19.67%   +0.01%     
==========================================
  Files         112      112              
  Lines       13713    13722       +9     
==========================================
+ Hits         2696     2700       +4     
- Misses      11017    11022       +5

Impacted Files	Coverage Δ
Source/Charts/Charts/BarLineChartViewBase.swift	23.61% <61.53%> (+0.17%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 04c5820...24275ae. Read the comment docs.

[liuxuan30]

What's this for? Any screenshot to demonstrate? From the code I don't see the difference for grid line in front or background?

[JacopoMangiavacchi]

Sure thing @liuxuan30 , an image is better than 1000 words ;-)

[liuxuan30]

😂 however I can't see the grid line difference?

[JacopoMangiavacchi]

Oh, maybe there's another way but I didn't found an existing settings for having the (white) vertical lines draw on top of the (blue/orange) horizontal lines like in the picture above. I just added this setting and in the draw() method I moved the drawing of these lines after the bar lines if this property is true.

[liuxuan30]

Are you saying the white vertical thin lines are grid lines? If so, I think I understand now. The bars are as if cut off into small rects by the grid line.

Well, I feel these grid lines are confusing - the grid lines are interfering reading the chart. Can you give some examples in real world to support this kind of grid lines?

[JacopoMangiavacchi]

Yes @liuxuan30 . I'm not a designer but as said I just had this UX request to implement a chart like this, with bars cutted off in small rect by the grid line. Other charts frameworks have this feature but I preferred to use Charts so I've implemented this.

[JacopoMangiavacchi]

Btw, of course this is just a new optional configuration parameter (drawGridLinesOnTopEnabled) where the default value (false) draw a traditional chart with grid lines below the bar line.

[liuxuan30]

Sure, I see. I will leave this PR to @danielgindi , as I'm not sure if we should add this. I am fine, but adding more 'switches' also needs to be careful

[jjatie]

Grid lines should be behind the data. If this is a requirement for your project, that's fine, but it's not something for the shared framework.