To add an annotation or comment to a chart you will want to start in the chart editor as shown below:
From here, you will see your chart displayed in the center of the screen. You will need to hover over any specific data point on the chart that would like to annotate.
When hovering over a data point, you will see "Click to Annotate" appear in the tool tip. Click the specific data point, and a text box will display where you can add up to a 100 character annotation.
Click "Save", and you will be on your way!
An example is shown below: