View source for OnGraphDraw/Label Minimum and maximum

This illustrative [[OnGraphDraw]] example labels the minimum and maximum of a graph. It is provided to help you learn how to create your own [[OnGraphDraw]] embellishments. 

<center>[[image:OnGraphDraw min and max.png]]</center>

== About the example ==
This example here is intentionally kept simple so that it is more of a tutorial that a fully robust example. In particular, we will not be worrying about our annotations falling off the edge of the visible area then the min or max occurs too close to the right edge, and we will not worry about the possibility of our annotations overlapping each other. 

== Before starting ==
Before you can use the [[OnGraphDraw]] attribute, you need to make it visible. You only need to do this once for a given model, by following these steps:
# Open the [[Attributes dialog]] by selecting '''Attributes...''' from the '''Objects''' menu.
# Ensure '''class=Variables''' is selected at the top.
# Click twice on '''OnGraphDraw''' so that a check mark appears by it.
# Press '''OK'''

== A variable with some data ==
You'll need some data for your plot. For some random data do this:
# Select '''Edit Time''' on the '''Definition menu''', and set its definition to <code>0..100</code>
# Create a variable, MyData, defined as: <code>[[Cumulate]]( [[Random]]([[Normal]](0,1,over:Time)), time)</code>

View the result and set its line style to points only. You should see something similar to the above plot, but probably with different random data, and without the minimum and maximum annotations yet.

== The annotation ==
I'm going to show you how to create a simple [[User-Defined Function]] that annotates the min and max of a graph. If this is something you do on a regular basis, you would file this function away in a library, and all you have to do is call this function from [[OnGraphDraw]]. Once you have this function, from the [[Object Window]] of your result variable, just set the [[OnGraphDraw]] attribute to
<code>
::Annotate_Min_And_Max(canv, info, roles, Self, Time)
</code>
Note that <code>canv</canv>, <code>info</code>, <code>roles</code> and <code>Self</code> are local variables provided automatically to the expression in [[OnGraphDraw]].

== The functions ==
Create a new [[UDF]] as follows:
<code>
:Function Annotate_Min_And_Max( canv, info, roles, y ; J : Index)
::Annotate_point( canv, info, roles, [[ArgMin]](y, J), [[Min]](y, J), "The minimum");
::Annotate_point( canv, info, roles, [[ArgMax]](y, J), [[Max]](y, J), "The maximum")
</code>

And create a second [[UDF]] that does the heavy lifting:
:Function <code>Annotate_point( canv, info, roles, x, y, label )</code>