Angle between two XY positions

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Will

I am having trouble getting the angle between two points. I have written this function (which has seemed to work for me in other languages, maybe not, i'm not sure now)

Function AngleBetween:Float(x1, y1, x2, y2)
       ax = x2-x1; ay = y2-y1
       ang# = 0
       If ax <> 0 Then
              ang = ATan((y1-y2) / (x1-x2))
       Else
              If ay > 0 Then
                     ang = 90
              Else
                     ang = 270
              End If
       End If
       If ax < 0 And ay < 0 Then
              ang = ang + 180
       End If
       If ax < 0 And ay >= 0 Then
              ang = ang + 180
       End If
       Return ang
End Function

So this function, basically, doesn't work, and I have no idea why. I know that its not working, because when I test it like this:

SetLineWidth(5)
SetColor(0,0,0)
ang# = anglebetween(512, 384, MouseX(), MouseY())
DrawLine(512, 384, 512 + 10000*Cos(ang), 384 + 10000*Sin(ang))

What I should see is a line, from the center of my screen (which is 1024x768) draw through the mouse, that follows it as I move my mouse around the screen. However what happens is strange, and hard to describe, i would recommend testing it yourself. It is as if some angles work, and others are snap-to fit on 45 degree increments. Please help me figure out how to do this, as I'm working on a school project, and its currently inhibited by this weird issue.