Charts: 饼图无法将孔颜色设置为nil

对于透明孔，建议：

*Note: Use holeTransparent with holeColor = nil to make the hole transparent.*```

但是，以下设置会导致崩溃：

pieChart.holeTransparent = true
pieChart.holeColor = nil

看一下实现，holeColor被显式地解包了，大概是因为检查了它之前的nil。 但是，显式展开不在检查范围之内：

if (holeColor !== nil && holeColor != UIColor.clearColor())
            {
                // draw the hole-circle
                CGContextSetFillColorWithColor(context, holeColor!.CGColor)
                CGContextFillEllipseInRect(context, CGRect(x: center.x - holeRadius, y: center.y - holeRadius, width: holeRadius * 2.0, height: holeRadius * 2.0))
            }

            // only draw the circle if it can be seen (not covered by the hole)
            if (transparentCircleRadiusPercent > holeRadiusPercent)
            {
                let alpha = holeAlpha * _animator.phaseX * _animator.phaseY
                let secondHoleRadius = radius * transparentCircleRadiusPercent

                // make transparent
                CGContextSetFillColorWithColor(context, holeColor!.colorWithAlphaComponent(alpha).CGColor)

                // draw the transparent-circle
                CGContextFillEllipseInRect(context, CGRect(x: center.x - secondHoleRadius, y: center.y - secondHoleRadius, width: secondHoleRadius * 2.0, height: secondHoleRadius * 2.0))
            }

临时解决方法：

pieChart.holeColor = UIColor.clearColor()