This appears to be an unknown issue with our charts.
I have forwarded this onto our development team to investigate further.
A work around solution would be to increase the maximum value that is initialised in your Y-axis' range like so:
SChartNumberRange *yNumberRange = [[SChartNumberRange alloc] initWithMinimum:@0 andMaximum:@20.1];
Let me know how you get on.