CPU Utilization in LineChartRenderer.swift

## What did you do?

Updated to Swift 4.2 compliant version of Charts.

## What did you expect to happen?

Swift 4.2 compliance

## What happened instead?

Everything went smoothly, except that the CPU utilization increased significantly from ~33% to ~90% while presenting a continuously updated line chart 15 times a second resulting in choppy visual performance.

However, I was able to find in **LineChartRenderer.swift** on line 632 one of the changes from the previous version was the removal of the OR test for `!dataSet.isDrawCirclesEnabled `for a continue statement. Which means even though the circles are not drawn, the process still falls through the continue statement executing the code. When I replaced the OR test the CPU utilization returned to where it was previously.

I returned this:
```
if !dataSet.isVisible || dataSet.entryCount == 0
{
continue
}
```

Back to this:
```
if !dataSet.isVisible || !dataSet.isDrawCirclesEnabled || dataSet.entryCount == 0
{
continue
}
```
to regain the performance.

Thanks.
Stoodwalk

## Charts Environment

**Charts 3.2.1/ **Version 3.2.1**, synced to [MPAndroidChart #f6a398b]
**Xcode version: 10.1
**Swift version: 4.2
**Platform(s) running Charts: iOS
**macOS version running Xcode: 10.13.6