In a different SCADA package, I once added notes functionality to a chart like this:
Create database table for your notes including timestamp, note contents, username, and any other information relevant to your process.
Set up a pen on the chart to query from the notes table - on my app I just select the value 0 for each row in the table to give a simple marker on the chart for each note event.
Set up a table/spreadsheet on the same screen as the chart with a query that is linked to the currently displayed time range on the chart, so the operator can read the notes - I kind of like this better than displaying the notes ON the actual chart, because to me that can get very cluttered in a hurry.