This is a convenience function to visualize value ranges (mean +/- std. deviation) with horizontal lines in a plot, typically one generated by
iso_plot_data. Considers aesthetics color and group as well as the facet variables to calculate averages within each panel, color and group. This leaves aesthetics fill and shape to be used for other purposes in displaying data points.
iso_mark_value_range(p, mean = TRUE, sd = 1:2)
logical (default TRUE), whether to show a linef or the value mean
which standard deviation intervals to show (mean +/- sd), by default +/- 1 and +/-2 population std. deviations. To omit these intervals, set