Tidy up / small issues

[johnmackintosh]

• Calculate y value once, up front to speed up plotting

• Consider calculating flag outside of the processing function

• verify (again) that there is only one set of limits per data point prior to plotting

• Check that there are sustained points before plotting and skip if necessary. Currently generates a warning when plotting

• Ensure that previous points do not get highlighted as per points on previous centre should not be highlighted #7

• match arg - make 'both' the default for direction

• Documentation

• Ensure upper limits capped at 100% for P chart

• P Prime and U Prime charts

• Duplicate centre lines on some plots

• add test dataset and examples