Bubble are not visible

> ℹ Please fill out this template when filing an issue.
> All lines beginning with an ℹ symbol instruct you with what info we expect. You can delete those lines once you've filled in the info.
>
> Per our [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md), we use GitHub for
> bugs and feature requests, not general support. Other issues should be opened on Stack Overflow with the tag [`ios-charts`.](https://stackoverflow.com/questions/tagged/ios-charts)
>
> Please remove this line and everything above it before submitting.

* [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?

ℹ
I am creating bubble chart for my data but the bubble are not visible on chart . But when I click on the point which I have given to draw the bubble then I noticed that bubble are there(**means able to see the boundaries of bubble after click l** ) but these bubble are not visible.

## What did you expect to happen?

ℹ Bubble should be there .

## What happened instead?

ℹ Then I try the same points(Dataset) on the sample application downloaded the I saw all bubble are correctly visible . I match my whole the code is excatly same as in the sample code still bubble are not visible .

## Charts Environment

**Charts version/Branch/Commit Number:**
**Xcode version: 10.2.1
**Swift version:5 .0
**Platform(s) running Charts: iPhone X Simulator
**macOS version running Xcode: ma OS Mojave 10.14.4

## Demo Project

ℹ This the attached file of code .
https://drive.google.com/file/d/1vBP2JNqyz1Gc3u-nAP0foTgvzIw10KXM/view?usp=sharing
@danielgindi @ParkinWu @liuxuan30 @chanil1218 @PhilJay @kemchenj