mouseY()
A Number system variable that tracks the mouse's vertical position.
mouseY keeps track of the mouse's position relative to the top-left corner of the canvas. For example, if the mouse is 50 pixels from the top edge of the canvas, then mouseY will be 50.
Examples

function setup()
size(100, 100)
describe("A horizontal black line moves up and down following the mouse's y-position.")
end
function draw()
background(200)
-- Draw a horizontal line that follows the mouse's y-coordinate.
line(0, mouseY, 100, mouseY)
end

function setup()
size(100, 100)
describe("A gray square. The mouse's x- and y-coordinates are displayed as the user moves the mouse.")
end
function draw()
background(200)
fill(0)
-- Style the text.
textAlign(CENTER)
textSize(16)
-- Display the mouse's coordinates.
text('x: '..mouseX..' y:'..mouseY, 50, 50)
end
Syntax
mouseY
Returns
Number: current y-coordinate of mouse position.