Arc-length plot bug

Created by: voisey

There appears to be a bug in the plot_arc_lengths action. See for example the first plot in this report: http://pcteserver.mi.infn.it/~nnpdf/validphys-reports/BmxSKTr7Twm-iG6A35gzyg==/ where the bug is that some of the points lie outside the error bars.