Bland Altman plot

Generates bland Altman plot

Function to generate Bland Altman plots. Barry Greene, September 2008
Bland, J.M., Altman, D.G. 'Statistical methods for assessing agreement ...
between two methods of clinical measurement'(1986) Lancet, 1 (8476), pp. 307-310.
Inputs: data1: Data from first instrument
data2: Data from second instument
Outputs: data_mean: Mean of data recorded by both instruments
data_diff: Difference of data recorded by both instruments
md: Mean difference
sd: Standard deviation of difference
Produces Bland Altman plot with mean difference and mean difference +/-
2*SD difference lines.