View source for Plot error bars

''New to [[Analytica 5.2]]''

Use this function in the [[OnGraphDraw]] attribute to plot error bars around data points. Error bars are vertical lines that extend above and below a data point to visually indicate a range of uncertainty. The annotation is usually applied to a line-symbol chart where only one line is present, or less commonly to a vanilla bar chart.
<center>[[image:Example of  Error Bars.png]]</center>

== Plot_error_bars( canv, info, roles, spread ) ==

To use, in the [[OnGraphDraw]] attribute:
* From the [[Attributes dialog]], turn on "Evaluate before drawing" AND "Evaluate after fully drawn".
* Calculate a spread for your variable. A common choice is: <code>[[SDeviation]](Self)</code>.
* Set the [[OnGraphDraw]] expression 
<code>
:[[If]] info[ [[OnGraphDrawItem]]='ViewMode']='Mean' [[Then]]
::roleChanges := [[Plot_error_bars]](canv, info, roles, phase, spread:[[SDeviation]](Self));

* Note: You can substitute your own spread expression.
* Note: Error bars with <code>spread:[[SDeviation]](Self)</code> are usually appropriate only from the Mean result view.

The annotation works with a linear or log-scaled continuous Y-axis showing the variable's mean value. It also works with Swap XY (in which case the error bars are horizontal).

== See Also ==
* [[Plot_point_labels]]
* [[Plot_solid_prob_bands]]
* [[Plot_Tukey_bars]]