<OTDataGraphable>
<dataNotes>
  <OTDataNote xVal="8">
     <text>This is the top of the first hump</text>
  </OTDataNote>
  ...
</dataNotes>
</OTDataGraphable>

I have thought before that maybe that should be associated with the
DataStore. This would allow the note to be seen in any visual
representation of the data. For example, if the data is represented in
a table then it would be nice to have notes like that next to the value
in the table.

In that case it would be something like:

<OTDataStore>
  <values>
    ....
  </values>
  <dataNotes>
    <OTDataNote xValue="8" yValue="10" xColumn="0" yColumn="1">
      <text>This is the top of the first hump</text>
    </OTDataNote>
    ...
  </dataNotes>
</OTDataStore>    

The xColumn and yColumn would default to 0 and 1 respectively.
if xValue and yValue are not present then the note is a general note
if xValue is present then the note if for all values at that xValue
if yValue is present then the note is for all values at that yValue
if they are both present then the note is for just that value.

Most of this data is not going to have two y values for the same x on
the same line, so proably the yValue attribute will never be used.

A flaw with this is that you can't put a note on several columns at
once. For example if you are collecting from 2 sensors and you want to
indicate that something happened at a particular time. There isn't a
valid syntax for representing this, so we'll have to just add multiple
notes if this is critical.