Drawing on the plot area

[korra]

Hi,
I'm trying to draw a line on the plot area.
I have modified the index.htm file adding the following code inside the function "updateTriggerSlider()":

var plot_canvas = $('#flot-overlay')[0];
var plot_context = plot.getContext('2d');

and then
plot_context.beginPath();//horizontal line
plot_context.lineWidth = 2;
plot_context.strokeStyle="green"; // Green path
plot_context.moveTo(0,75);
plot_context.lineTo(250,75);
plot_context.stroke(); // Draw it

When I run the file the browser tell me that "flot-overlay" does not exist. Infact it is not declared in the index.htm (I found flot-overlay looking at the chrome debug tools)
For example how is generated the green line for the trigger in normal mode?
I cannot find it all around the index.html or the .c/.h files.

Thanks,
Corrado