Modify offsetForDrawing and draw method in BallonMarker swift

[oxape]

The MarkerView can't indicate actual position of data.
Before

Now after modification.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1ed1561). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1741   +/-   ##
=========================================
  Coverage          ?   18.68%           
=========================================
  Files             ?      123           
  Lines             ?    14052           
  Branches          ?        0           
=========================================
  Hits              ?     2626           
  Misses            ?    11426           
  Partials          ?        0

---

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 1ed1561...0a56492. Read the comment docs.

[liuxuan30]

thanks @oxape! Please wait for a while as the review process could take time.. As you can see we are short handed and lots of questions and bugs to fix..

[danielgindi]

This is pretty. Can't wait to merge this :-)

[oxape]

I am very much looking forward

[jjatie]

Please rebase master

[liuxuan30]

seems he deleted that branch..

[jjatie]

I guess I'll reimplement