plot.optint_by_group: Plot optint object, by group
In optinterv: Optimal Intervention

Produce variables importance plot from an optint_by_group object. This plot has several features:

1.: Estimates here are E(X | I=1) - E(X | I=0) and not cdf distances.
2.: Star is added to variable name if there is a significant difference between at least two groups.
3.: Estimates are standardized before they plotted (so different set of variables will have different standardization factor.)

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## S3 method for class 'optint_by_group'
plot(x, plot.vars = "sig", graph.col = NULL,
  alpha = 0.05, ...)

`x`	an optint_by_group object.
`plot.vars`	which variables to plot? either a number (n) - indicating to plot the first n variables, "sig" (default) - plot only significant variables (here significant means that variabe is signifcant for all groups, or that there is significant heterogeneity), or a vector with names of variables to plot.
`graph.col`	graph color/s.
`alpha`	significance level for the confidence intervals. also used in order to determine which variables are significant.
`...`	additional arguments.