A data frame containing either derived_posteriors.sso or a good
subset of it. This can be an element of the list created by the the
SSgetMCMC function.
add
TRUE/FALSE option to add results to an existing plot.
nsexes
Number of sexes in the model (should match model values but is only
used in the title).
shift
Optional adjustment to the x values to avoid overlap of intervals
when overplotting on an existing plot.
fleetname
Optional input to make the title better. Default will be something
like "Fleet 1", using the numbering from the model.