BSChart graph do not support negative y-scale values

[jumbopilot]

The line graph won't simply show as soon as negative values are part of BSChartItem()

debug message:
BSChart[22615:672934] [Unknown process name] Error: this application, or a library it uses, has passed an invalid numeric value (NaN, or not-a-number) to CoreGraphics API and this value is being ignored. Please fix this problem.

Probably related to issues#3

[jumbopilot]

The required code change to resolve the issue is:

let factor = height / abs(CGFloat(max - min))
instead of
let factor = height / CGFloat(abs(max) - abs(min))

in draw... functions of BSChartView.swift to properly define the y-scale value range (factor).