Equations of straight line graphs via changing coordinates

Exploring the effect of changing axes on the equation of a straight line
This graph shows a point on a line of gradient m passing through the point . You can control the gradient of the line with the slider, you can move the point and you can move the red point. What condition do the coordinates of the red point satisfy in the blue axes (the (X,Y) coordinates)? How are the blue coordinates (X,Y) related to the black coordinates (x,y)? What condition do the coordinates of the red point satisfy in the black axes (the (x,y) coordinates)?