Draw pixel and alpha channel

[Tomaaz]

Here is the Bubble Universe example. It seems that alpha element doesn't affect drawing single pixels:

Code:

set window "Bubble Universe", 512, 512
r = 6.283185307179586 / 235
mw = 512
mh = 512
hw = mw / 2
hh = mh / 2
x = 0
v = 0
u = 0
t = 0
k = rnd(190) + 64
set redraw 0
while 1
    set color 0, 0, 0, 255
    draw rect 0, 0, 512, 512, 1
    for i = 0 to 255
        for j= 0 to 255
            u = sin(i + v) + sin(r * i + x)
            v = cos(i + v) + cos(r * i + x)
            x = u + t
            set color i, j, 255 - i, k
            draw pixel int(hw + u * hw * 0.4), int(hh + v * hh * 0.4)
        next
    next
    redraw
    t = t + 0.01
    k = rnd(190) + 64
wend

 
Is this a bug? I've tried draw pixel, set pixel and even both.