Line Chart Crashes if it only has equal values for a line

[colindownes]

Hi,

I have a chart with a single line I set startFromZero as true I set data values to [0,0]

you then get

*** Terminating app due to uncaught exception 'CALayerInvalidGeometry', reason: 'CALayer position contains NaN: [20 nan]'

Caused by

(CGFloat)_positionYForLineValue:(CGFloat)value { CGFloat scale = (value - [self _minValue]) / ([self _maxValue] - [self _minValue]);

because ([self _maxValue] - [self _minValue]) == 0

I can try and add a possible pull request when I get chance. Thanks!