There are times when it is useful to add additional points of reference to a chart to provide more context for the visualization. Tableau makes it easy to add reference lines, trend lines, box plots, and clusters from the analytics tab in the sidebar. The analytics tab was introduced in Tableau 9.

Appendix C, Working with Tableau 10 introduces the different types of reference components supported in Tableau - line, band, distribution, and box plot. You can also check out the Tableau online documentation, found at http://bit.ly/tableau-reference-types, for more details.